This is a great question. However, it is very broad. In a fast moving world, twenty years is a lot of time for change to happen. However, I would say that the most important event in the past twenty years happened on 9/11. The terrorist attacks on U.S. soil not only changed domestic policy, but also foreign policy.
Perhaps the most noteworthy point is the Patriot Act. It was signed by President Bush on October 26, 2011. The full name is called USA PATRIOT, which is an acronym meaning: Uniting, Strengthening, America, Providing, Tools, Required, Intercept, Obstruct, and Terrorism.
By the Patriot Act, the U.S. is far more proactive when it comes to monitoring any suspicious activity - domestic or foreign. Remember the words of President Bush that the U.S. would not put up with nation who even harbor terrorists. Since then, the U.S. involvement in the Middle East has increased and several regimes have been put down.