After the fall of the Soviet Union in the 1990's, U.S. President George H. W. Bush declared a New World Order. He wanted to bring Russia into a New market economy where organizations were ran by the wealthiest nations.
There was still lasting anger towards the U.S. after the Cold War. There were some countries who also opposed the U.S. for being the worlds sole superpower. In addition, many muslim nations many Muslim nations were hostile due to the U.S. supporting Israel every year.
After the Cold War period there was a Waring period between sovereign states. Groups emerged that weren't part of any government that controlled and killed civilians called terrorist networks. There were several terrorist attacks around the world to intimidate countries and get their message across.
The September 11 attacks originated from Bin Laden's camp in Afghanistan, which was under a group called the Taliban. When they refused to turn him over, the U.S. used force to capture him. The U.S. invaded Afghanistan and took overthrew the government. They later found Bin Laden and killed him.
As the war in Afghanistan was heating up, President Bush claimed that Iraq'a leader was connected to the attacks and that the country was harboring weapons of mass destruction. The U.S. invaded Iraq with no evidence and killed their leader. This tarnished the United States reputation and sympathy after 9/11.
The defeat of the Taliban and the death of Bin Laden did not end terrorism however. New leaders and groups emerged. Many countries began to military spendingd to counter these threats.