The 'Invasion of Luxembourg' was part of 'Gelb', the German invasion of the Low Countries (the Netherlands, Belgium and Luxembourg) and north-eastern France (10 May 1940).

The invasion lasted just one day as, in the face of only light resistance, German troops quickly occupied Luxembourg. The Luxembourg government and the Grand Duchess Charlotte managed to escape the country and a government-in-exile was created in London.

On 1 September 1939 Germany invaded Polandin 'Weiss' (i), thereby triggering World War II. This put Luxembourg’s Grand Ducal government in a delicate situation: on the one hand, the population’s sympathies lay with the UK and France, and on the other and as result of the country’s policy of neutrality since the Treaty of London in 1867, the government adopted a careful stance of non-belligerency toward its neighbours. In accord with the treaty’s restrictions, the only military force Luxembourg maintained was its small Volunteer Corps under Capitaine Aloyse Jacoby, reinforced by the Grand Ducal Gendarmerie under Capitaine Maurice Stein. Together these two units constituted the Corps des Gendarmes et Volontaires under the operational control of Major-Commandant Émile Speller.

At 12.00 on 1 September Radio Luxembourg announced that in order for the country to remain unambiguously neutral it would cease broadcasting except for a daily 20-minute message at 12.00 and in the evening for government announcements. For the rest of the month, the government supplied full transcripts of its broadcasts to the foreign legations in the country. Later on 1 September, several German stations posed as Radio Luxembourg by broadcasting on Radio Luxembourg’s wavelength to make what were, in the opinion of the US chargé d’affaires, 'grossly un-neutral announcements'. On the evening of 21 September, the Grand Ducal government suspended all broadcasts.

On 14 September the VolunteerCorps was bolstered by the addition of a 125-strong auxiliary unit. German military manoeuvres and river traffic made the population increasingly nervous, so in the spring of 1940 fortifications were erected along the borders with Germany and France. This so-called 'Ligne Schuster', so named after its chief constructor, comprised 41 sets of concrete blocks and iron gates, 18 bridgeblocks on the border with Germany, 18 roadblocks on the border with Germany, and five roadblocks on the border with French. As the Corps des Gendarmes et Volontaires had no pioneer unit, construction was contracted to civilian engineers, while technical advice was sought from the French. A series of nine radio outposts was established along the German border, each manned by gendarmes, with a central radio receiver in Stein’s office near the volunteers' St Esprit Barracks in the capital. On 4 January 1940, the cabinet convened under the Grand Duchess Charlotte and outlined steps to be taken in the event of a German invasion. The grand duchess decided that, if possible, she and the government would flee abroad in the event of an attack to advocate for the country’s sovereignty. During World War I, the grand duchess’s elder sister, the Grand Duchess Marie-Adélaïde, had elected to remain during Germany’s occupation of the country, bringing the monarchy into disrepute. Charlotte wished to avoid such problems. The government moved some of the country’s gold reserves to Belgium, and began stockpiling funds in its Brussels and Paris legations in the event it was forced to flee in the face of German attack. The Paris legation was also given a sealed envelope detailing a formal request of military assistance from the French government in case communications were severed by an invasion.

After several false alarms in the spring of 1940, the probability of a military conflict between Germany and France grew. Germany stopped the export of coke for the Luxembourgish steel industry. Abwehr intelligence agents under Oskar Reile infiltrated the country in the guise of tourists, a fact which was observed by Capitaine Fernand Archen, an undercover senior French intelligence officer in Luxembourg City, posing as a wine merchant. Archen reported his findings to his superiors at Longwy on 7 May, understanding that the agents were to be used to seize key bridges over the Sauer, Moselle and Our rivers. The Luxembourg authorities also took note of the German activity, and Stein worked to stop the German activities. On 3 March, the French ordered Général d’Armée Charles Marie Condé's 3ème Armée to occupy Luxembourg in the event of a German attack.

On the evening of 8 May, the Grand Ducal government ordered for the first time that all doors of the 'Ligne Schuster' be closed at 11.00 and remain so regardless of circumstance until 06.00 on the following morning. Throughout the day the Luxembourg authorities witnessed much less activity on the far side of the border and made no reports of tank or machine gun movements. During the afternoon of 9 May, a French intelligence officer stationed in Clervaux witnessed German troops preparing pontoon bridges on the Sauer river. He attempted in vain to contact Archen, and resorted to making a direct phone call to his superiors at Longwy. On this same day, a German national working in Luxembourg as a gardener and a member of the German fifth column warned his Luxembourg employer, Carlo Tuck, that an invasion was impending, and Tuck passed the warning to government officials. Late in that evening, the Grand Ducal government came into possession of a document from a German divisional command. Dated 23 April 1940, it detailed the orders of the division’s chief-of-staff to various units to occupy strategic points within the country. The Grand Ducal government put all border posts and Grand Ducal gendarmerie stations on full alert. In Luxembourg City, gendarmes mobilised to defend public buildings and despatched vehicle patrols to arrest fifth columnists. The economic councillor and the chancellor of the German legation were detained for questioning in regard to allegations that they had used legation cars to organise subversive activities within the country. Since an invasion had not yet occurred, these Germans still enjoyed diplomatic privilege and the police were forced to release them. One group of fifth columnists was arrested while attempting to reach the legation. Meanwhile, Archen had received his subordinate’s report, but by that point, he had been told by informants in the Gendarmerie that shots had been exchanged with German operatives at a remote farm near the Moselle river. At 11.45 on 9 May Archen radioed Longwy with 'Reports of important German troop movements on the German-Luxembourg frontier'. Throughout the night Archen’s messages became more and more frantic. Two Luxembourg customs officials at Wormeldange heard horses and soldiers across the Moselle river, but were unable to make out the Germans' activities as a result of heavy fog.

At about 00.00, Capitaine Stein, Minister of Justice Victor Bodson and Police Commissioner Joseph Michel Weis held an emergency meeting. Bodson requested that the capital be reinforced by gendarmes from the south, and told Weis to forward this information to the capital’s district commissioner to give the necessary orders. Weis later tried to contact the district commissioner by telephone, but failed to reach him, and no reinforcements ever arrived. A short time later the gendarmes at Diekirch were ordered to patrol the local railway bridge and be wary of unfamiliar persons. The Luxembourg authorities received the first reports of exchanged fire at about 02.00 on 10 May, when two gendarmes were ambushed near the German border by plain-clothes agents. The Germans retreated to the Fels mill near Grevenmacher and around 20 volunteer soldiers were despatched to arrest them. The government then ordered the locking of all steel doors along the border. At 02.15, soldiers stationed in Bous were attacked by Germans in civilian clothes. One soldier was badly injured, as was one German, who was then detained. Shortly thereafter a gendarmerie lieutenant and his chauffeur were ambushed and exchanged fire with German-speaking cyclists, but no one was hurt. Fifth columnists successfully severed the telephone wires between the capital and the border posts, forcing the gendarmes to communicate via short-wave radio. German agents gradually seized the radio stations: the last post to fall, in Wasserbillig, transmitted until the Germans breached the operating room.

The steel doors of the 'Ligne Schuster' were ordered closed at 03.15 on 10 May following reports of movement of German troops on the eastern side of the Our, Sauer and Moselle border rives. At 03.30 the Luxembourg authorities released interned French pilots and German deserters. The royal family was evacuated from its residence in Colmar Berg to the grand ducal palace in Luxembourg City, and about minutes later, at dawn, German aircraft were sighted overflying Luxembourg City toward Belgium.

The German invasion began at 04.35 when Generalleutnant Friedrich Kirchner’s 1^st Panzerdivision, Generalleutnant Rudolf Veiel’s 2^nd Panzerdivision and Generalleutnant Ferdinand Schaal’s 10^th Panzerdivision crossed the border at Wallendorf Pont, Vianden and Echternach respectively, crossing the tank traps of the 'Ligne Schuster' by means of wooden ramps. Fire was exchanged, but the Germans did not encounter any significant resistance except for the destruction of some bridges and the presence some land mines, for most of the Volunteer Corps remained in its barracks. The border was defended only by soldiers who had volunteered for guard duty, and by gendarmes. A few Germans secured the bridge at Wormeldange and captured the two customs officers there, who had demanded that they halt but refrained from opening fire. The partly demolished bridge over the Sauer river at Echternach was quickly repaired by engineers of the Infanterieregiment 'Grossdeutschland', opening the way for the passage of the 10^th Panzerdivision. German warplanes flew overhead, heading for Belgium and France, though some stopped and landed troops within the country.

Archen repeatedly alerted his superiors at Longwy of the invasion, but his reports did not reached the 3ème Armée at Metz. Condé was uncertain about the situation and at 05.30 despatched aerial reconnaissance units to investigate, and at 06.00 Général de Division Robert Marie Eduard Petiet’s 3ème Division Légère de Cavalerie was ordered to intervene.

Telephone and radio messages from the border posts to the Gendarmerie and Volunteer Corps headquarters informed the Luxembourg government and Grand Ducal court of the invasion. Foreign Minister Joseph Bech, in the presence of Prime Minister Pierre Dupong, attempted to contact the German ambassador at the legation and at his private residence, but was informed that he was present at neither. At 06.30 the majority of the government, including Dupong and Bech, departed the capital by motorcade to the border town of Esch. Bodson remained behind at the St Esprit Barracks to monitor the situation. In Esch a group of 125 German special operations troops had landed by Fieseler Fi 156 Storch single-engined lightplane with orders to hold the area until the main invasion force arrived. A gendarme confronted the soldiers and asked that they leave, but was taken prisoner. The government motorcade encountered a roadblock at a crossroads manned by German units, and was forced to detour through the countryside to avoid capture. The French ambassador, Jean Tripier, followed the government party but was stopped by the Germans and forced to return to the capital. The Belgian ambassador, Kervyn de Meerendré, was also stopped by German soldiers at the border and ordered to turn back, as was the Luxembourg minister of education, Nicolas Margue, who had attempted to escape by taxi. Bodson later fled the capital and, having memorised many of the secondary roads, was able to avoid German roadblocks and navigate his way to France.

Following consultation with her ministers, the Grand Duchess Charlotte decided to abandon the palace. Accompanied by her husband, Prince Felix, her mother, the Dowager Grand Duchess Marie Anne, and members of the Grand Ducal suite, she departed for the border village of Redange. and, after a brief stop, crossed the border into France at 07.45. Meanwhile, the Hereditary Grand Duke Jean and two of his sisters, accompanied by an aide-de-camp, Guillaume Konsbruck, were to wait at the border for confirmation of occupation. At about 08.00, the prime minister and his entourage passed over the border before making contact with French troops at Longlaville. Last-minute telephone calls with Luxembourg City revealed the capital to be completely surrounded.

Charlotte’s party was able to link with the government motorcade at Longwy. Meanwhile, the car of Jean’s party was strafed by a German aeroplane while stopped at a cafe. Near Esch, the group was delayed by a German roadblock, and escaped when its chauffeur drove straight through the soldiers. The party ultimately joined Charlotte and the Grand Ducal government at Ste Menehould.

At 08:00, a French force on the form of elements of the 3ème Division Légère de Cavalerie, supported by the 1ère Brigade des Spahis under Colonel Jouffault and the 2ème Compagnie of the 5ème Bataillon Cuirassée, crossed the southern border to conduct a probe of the German forces; these units later retreated behind the 'Ligne Maginot' after five Spahis had been killed. Air Marshal A. S. Barratt, commander opf the British air forces in France, was impatient with the reluctance of the French air force to undertake air attacks, and ordered a flight of Fairey Battle single-engined light bombers from No. 226 Squadron to attack German tank columns.The Battle bombers were unescorted and encountered heavy anti-aircraft fire, and while most were damaged by Flak but managed to escape, one received a direct hit and crashed near Bettendorf. German soldiers pulled the three injured crew from the burning wreckage, and one of the men later died in a local hospital.

The Gendarmerie resisted the German troops, but to little avail, and the capital city had been occupied before 12.00. The Gendarmerie chain of command in the south was thrown into disarray by the influx of refugees and the arrival of German and French troops. Most gendarmes escorted refugees over the border, while some abandoned their posts and fled to France. The total Luxembourgish casualties amounted to six gendarmes and one soldier wounded, while 22 soldiers (six officers and 16 non-commissioned officers) and 54 gendarmes were taken prisoner.

By the evening of 10 May, most of the country, with the exception of the south, had been occupied by German forces. More than 92,000 civilians fled from the canton of Esch sur Alzette as a consequence of the advance: 47,000 left for France, and 45,000 poured into the central and northern parts of Luxembourg.

On 11 May the Grand Ducal government reached Paris and installed itself in the Luxembourg legation. Fearing German air attack and finding the small facilities unsuitable, the government moved farther to the south, first to Fontainebleau and then Poitiers, before later moving to Portugal and thence the UK before finally settling in Canada for the duration of the war. In exile, Charlotte became an important symbol of national unity. The only official representative left behind in Luxembourg was Albert Wehrer, head of the Ministry of State Affairs, as well as the 41 deputies. By the end of May, Wehrer and several high-ranking functionaries had established a provisional 'Administrative Commission' to govern Luxembourg in place of the Grand Ducal family and the other ministers.

In the days after the invasion, Luxembourg officers walked about the capital freely, though the regular soldiers were mostly confined to their barracks. Speller was briefly incarcerated by the Gestapo, though he was later released under close supervision.