Final answer:
The Bush Doctrine led to a proactive and unilateral American foreign policy with the use of preemptive strikes against perceived threats, exemplified by the 2003 invasion of Iraq and a shift away from international treaty commitments.
Step-by-step explanation:
The Bush Doctrine significantly impacted the American foreign policy post-9/11. It represented a shift from a noninterventionalist approach to a proactive foreign policy that included preemptive strikes and unilateral action when perceived threats to national security were identified. The adoption of this doctrine led to major foreign policy decisions such as the invasion of Iraq. This proactive stance led to contention both domestically and internationally as the U.S. moved to act without broad international support or direct provocation, redefining its engagement with the world.
The doctrine was defined by the idea that the U.S. had the right to protect itself pre-emptively from terrorist attacks by engaging in wars or overthrowing hostile governments. This was starkly exemplified in the invasion of Iraq in 2003. The Bush administration's argument was largely based on the belief that Saddam Hussein posed a significant threat due to his alleged possession of weapons of mass destruction, although this intelligence was later proven incorrect.
During the Bush presidency, there was also a notable shift away from diplomatic engagement and treaty commitments, such as environmental agreements and arms control treaties. The Bush Doctrine, therefore, represented a shift towards a more aggressive and unilateral American foreign policy.