Afghanistan
The "Bush Doctrine" was a foreign policy viewpoint that came from so-called "neoconservatives" among President George W. Bush's advisers -- especially some who had also served in the administration of his father, President George H.W. Bush. In the wake of the 9/11 attacks, there was a desire to push American values and not be shy about doing so with the use of American military might. Sometimes referred to as the "Bush Doctrine," the core ideas were that the United States could pursue foreign policy goals on its own (without need for United Nations partnerships), that preemptive strikes were allowable against countries that harbored terrorists, and that regime change for the sake of promoting democracy was a good strategy.
Because Afghanistan was where the terrorists that launched the 9-11 attacks had bases of activity, going to war in Afghanistan was seen as justified. The war in Afghanistan continues to be a problematic matter today, many years later.