Answer: The Middle East
Explanation: Since the end of the Cold War, the United States has been involved in various regions across the world, including the Middle East, Asia, and Europe. However, the region where the United States has been most involved since the end of the Cold War is arguably the Middle East.
Following the Gulf War in 1991, the United States has been involved in numerous military interventions and operations in the region, including the wars in Afghanistan and Iraq, the ongoing conflict in Syria, and military operations against terrorist organizations such as Al-Qaeda and ISIS.
The United States has also been involved in diplomatic efforts to promote peace and stability in the region, including the Israeli-Palestinian peace process and negotiations with Iran over its nuclear program.
Overall, the United States' involvement in the Middle East has been driven by various factors, including concerns about terrorism and regional stability, as well as economic and strategic interests such as oil and access to important waterways like the Persian Gulf.