Fall of Singapore / Conquest of Singapore
Issued in Japan, not in the occupied territory.

A commemorative card was known to exist with cancellation date of 21 February 1942.
 

SC#	Mi#	Face Value	Color		Print Run
B4	310	2 + 1 sen	Carmine Red	General Maresuke Nogi	5,680,000
B5	311	4 + 2 sen	Dark Bluish Green	Admiral Heihachiro Togo 27/01/1847 - 30/05/1934	5,610,000

Text from Wikipedia

Ministry of Communications of Japan, which had expected the Singapore fall, the face value (with donations stamps for letters and military spending donations of "Singapore fall" to 2 sen and 4 sen of Definitive stamp at the time (Nogi Maresuke and Togo Heihachiro) version in the 添刷 plus print (edition of characters on the plate of the existing stamp were added of), a commemorative stamp in which the so-called non-Overprint) to be applied to print stamps on February 16, 1942 immediately after the fall issue was. This stamp is called from production on the assumption that strategy is successful in Kigensetsu of February 11 had already completed to dispatch to the post office. Also Manchukuo in the same year March 1 has issued a commemorative stamp that was grazing in the Definitive stamp.

シンガポール陥落を予想していた日本の逓信省は、当時の2銭と4銭の普通切手（乃木希典と東郷平八郎）に「シンガポール陥落」の文字と軍事費募金のための額面（寄附金付切手）の版を加えた印刷（既存切手の版に文字の版を加えた添刷で、印刷済みの切手に施すいわゆる加刷ではない）をした記念切手を陥落直後の1942年2月16日に発行した。この切手は2月11日の紀元節に作戦が成功することを前提に製造から郵便局への発送まで既に完了していたという。また同年3月1日に満州国は普通切手に加刷した記念切手を発行している。