In legal|lang=en terms the difference between annexation and annexation

is that annexation is (legal) a legal merging of a territory into another body while annexation is (legal) a legal merging of a territory into another body.

As nouns the difference between annexation and annexation

is that annexation is addition or incorporation of something, or territories that have been annexed while annexation is addition or incorporation of something, or territories that have been annexed.