The US' barrier to exploitation was on the gold clause. If we printed too many dollars, then other countries could do a gold call with the increasingly worthless dollars, get valuable gold in exchange, take the currency out of play, and the issue would be fixed. So punishment and self interest in a quite well designed system, kind of. The problem is we just printed a bunch of money anyhow and then when France decided to make a gold call with their reserves, we shrugged, defaulted, and just broke the agreement.
So this started the modern economic era in 1971 where currencies became completely detached and free floating. At that point about 85% of global reserves were USD. Countries began rapidly dumping the dollar and USD inflation began skyrocketing. In 1980 inflation in the US was at 13.5%! This era continued til around around 1990 with the USD falling to 47% of reserves, but then in 1991 the USSR collapsed leaving the US not only as the sole 'king' of the world, but also to former Soviet nations dollarizing once the dust had settled. This continued up til 2001 (USD at 72%) when there was both the dotcom bubble and the clear rise of China and Russia as global powers. We've now been on a steady decline since, with the USD currently down to about 56% of reserves.
Originally, yes. The US dollar did become a reserve currency because because of the circumstantially huge negotiating power of the USofA, however, as it stands now, the negotiating power is dependent on the dollar being reserve currency. The US is running on a consistent trade deficit, which is supported by the dollars covering economic growth outside of US.
When self-supporting systems are thrown off-balance, the sign in the feedback loop tends to reverse.
It is becoming obvious for gulf countries that US military protection is worth nothing.
From what I hear, what Trump did with Iran was burn through the US stockpile of certain fancy special-purpose wepons you're really only meant to use to punch a hole for your conventional forces to enter by. Trump is not willing to send in those conventional forces, because many of those personel will die. Trump may not care about the lives of the US personel per se, but with the mid-terms coming up he will care about the impact of those deaths on the election result.
Also: Gulf Wars I and II had a lot of build-up and prep, moving forces into the area; this was diving straight in because someone (my Israeli namesake) flattered Trump's ego.
Now I also think it's not even controversial to say that drone warfare is radically changing the nature of engagements. I don't believe the US could take on present-day Iran, owing to Iran's production of militarily relevant quantities of cheap and good-enough drones; a saturation attack can drain the defensive capabilities of warships, and while the exact performance is classified, a slow upgrade cycle would probably allow a saturation attack to overwhelm the *sensor* capabilities from not too long ago.
Laser defence systems may change that, but those aren't all-weather solutions, and the sea is the exact worse case for this. And I don't know if anyone's thinking about saturation attacks with submersible drones.