We are on the brink of financial collapse. The US dollar is long overdue for hyperinflation and we can no longer afford policing the world and nation building. USD has been a fiat currency since the Nixon Shock.
The real reason for our involvement in Iraq, Syria, Libya, and soon, Iran, is that we are trying to maintain demand for the US dollar by engaging in military intervention with nations that refuse to sell their oil to the US in US currency (the Petrodollar system). The reason is certainly not for humanitarian reasons, as our government has propped up dictators in the Middle East over the past decades.
We think that we're protecting our security, but actually we are creating more enemies. The real threat to our security is our huge debt, not terrorists.