Complex sentence: The Revolutions of 1989 and the dissolution of the Warsaw Pact in 1991 removed the "de facto" main adversary of NATO and caused a strategic re-evaluation of NATO's purpose, nature, and tasks, and their focus on the continent of Europe.
Split into simpler sentences: The Revolutions of 1989 and the dissolution of the Warsaw Pact in 1991 removed the "de facto" main adversary of NATO. This caused a strategic re-evaluation of NATO's purpose, nature, and tasks, and their focus on the continent of Europe.