This was a German naval minelaying operation in the Kvarner gulf of German-occupied Yugoslavia to block Allied access to the Bay of Fiume (18 March 1944).
The Germans had converted the seized Italian transport Ramb III into the minelayer Kiebitz, and it was this vessel which was entrusted with the task of strengthening the existing Italian-laid minefields between the Istrian peninsula and the island of Cres.
Escort for the minelayer was provided by the ex-Italian torpedo boats TA 21 and TA 36, and ex-Italian corvette UJ 205, which were to escort Kiebitz to Fiume after she had completed her task and then make for Senj to escort landing craft of the 771st Pionierlandungsbataillon during 'Illusion'.
At 20.25 on 18 March TA 36 struck an Italian mine near Plomin bay, losing her bow and then sinking. The torpedo boat’s captain transferred to UJ 205 and decided to terminate the operation and return to Trieste, while TA 21 took 11 rescued seamen to Pola.
This meant that 'Illusion' had to be undertaken without the naval escort which could have blocked the resulting escape of the partisans.