'Sisak' was a Croat operation against the partisan forces of Marshal Josip Broz Tito in the northern part of the puppet state of Croatia within German-occupied Yugoslavia (23 March 1944).

After Usta�e intelligence had reported that the Western Group of Partisan Detachments was arriving in the area of Bistrica, an operation against the group was planned with a start time of 07.00 on 23 March 1944. By the time the operation began, however, the Western Group of Partisan Detachments had already moved in its capacity as the vanguard for 32^nd �Zagorje� Division, which was advancing toward Zagorje.

The Usta�e attack encountered the �Matija Gubec� Brigade, which had thought the area was safe, and as a result the Usta�e�s surprise attack on the brigade headquarters resulted in several partisan deaths, including those of the brigade commander, Mijo Bobetko, the chief medical officer and another 11 partisans; another three partisans were wounded.

The Usta�e force involved in the undertaking totalled about 1,000 men led by Nikola Ore�ković, commander of the Fast Battalion of Usta�e Pukovnik Ante Mo�kov�s Usta�e Poglavnik Bodyguard Brigade. Centred on the Fast Battalion, the force had six armoured vehicles, two machine gun platoons, a mortar half-platoon, an anti-tank gun half-platoon, a headquarters company, a grenadier company, a platoon from the Zlatar garrison, the 10^th Company and 12^th Company of the PZUV (Ustase militia traffic control brigade) from Oroslavlje, and the 2/1^st Regiment of the Usta�e Poglavnik Bodyguard Brigade.

The Usta�e reported its losses at two men killed and nine wounded, together with two motorcycles and three small arms, and claimed partisan losses of 147 killed and five taken prisoner, together with 42 rifles and nine horses. The partisans reported their casualties as 15 killed and six wounded, and estimated Usta�e losses at about 100. Each side had failed in its intelligence assessment of the other, resulting in confusion and a failure to exploit the circumstances. The subsequent �Sisak II� operation was launched in April.