Austria, World War II. Some people saw annexation, others saw reunification, others still saw a bunch of cowards surrendering without firing a single shot.

With all of these things, context matters. If memory serves, Austria being an independent state was one of the terms of the Treaty of Versailles, so if that had been the last act instead of the opening chapter, there still would have been hell to pay. Perhaps not on a par with Caesar crossing the Rubicon, but definitely starting something that requires a resolution.