Perhaps it's a sign of human progress* that in the Twentieth Century a lot of invaders felt they needed some good excuse to stage an invasion of a foreign country. Putin has continued that tradition. Malcolm Jones lists a few others.
*Or maybe not. If I recall correctly, Marcus Crassus came up with some cockamamie excuse for his ill-fated invasion of Parthia - despite a peace treaty and specific failure of the Roman Senate to authorize it.