During the Cold War, the challenges to the US were obvious. The US had to contain communism and work to spread democracy and capitalism. After the Cold War ended, things became more confused. Now, the US had to deal with a changing world and with threats from different places. The main source of problems at first came from the Middle East. This started with Iraq's invasion of Kuwait that led to the Persian Gulf War in 1990 and 1991. Later on, Islamist terrorism emerged from this region. At the same time, challenges arose in places like Africa and the Balkans where the US had to decide what to do about humanitarian crises that came with ethnic cleansing. Finally, there has been the rise of China to deal with.
In short, the challenge went from being an obvious challenge from one main source to being multiple challenges of different sorts coming from different places.