[bhpm]

The post WWII peace structure was based on an agreement between the US and other countries: we pay you money, we patrol the shipping lanes with our navy which is the only one left, and you side with us against the Soviet Union.

Once the Soviet Union imploded, there was little reason for this to be maintained and, sure enough, here we are.

[pjc50]

The USSR was never the only threat in the world, as we can see here. There's an awful lot of neutral shipping going through the Suez canal as well.

The US has been striking a very awkward balance between "we want our allies to spend more on their military and do more of their own defence" versus "but not enough to be an independent power". Resulting in odd things like the UK "independent" nuclear deterrent that has to be returned to the US for servicing, or the JSDF "helicopter carrier" (definitely NOT an aircraft carrier, despite the big long flat deck, because that would be prohibited under treaty).