 The postponement of Barbarossa from the initially planned date of 15 May to the actual invasion date of 22 June 1941 ( a 38 @-@ day delay ) occurred for a number of reasons . Most importantly , the Balkans Campaign required a diversion of troops and resources that hampered preparations , and an unusually wet winter kept rivers at full flood until late spring . The full floods could have discouraged an earlier attack , even if it was unlikely to have happened before the end of the Balkans Campaign . 
