For example, it happened in Iraq when the U.S. eliminated the people holding power, Hussein and the Baathists, and didn't have realistic plans for anyone to seize it. Iran, Shiite religious authorities, and Sunni insurgents all jumped for it. The mess still hasn't been resolved.
It's happening on a much larger, more significant scale globally as the U.S. withdraws. Others aren't sitting still; they are coming in and seizing the power for themselves. It is very hard won and the most valuable thing in the world; who would walk away from it?