There is an American Empire of a sort, but it's not the traditional kind. It's primarily cultural and financial, with military backup. They have taken Richard von Kuehlmann's concept of "limited sovereignty" to heart, it's a complex beast: institutions, international treaties and law, an endless stream of media and of course the petrodollar. There is little direct rule, as in a traditional setup, subject nations without the military power to face off with America may "rule" themselves, as long as that rule doesn't take them in a direction that does anything to upset the American objective. Or you find yourself being invaded to install freedom and democracy, or overthrown in a suspicious coup, or starved with sanctions.
You might say there are currently only 3 sovereign power blocks left on Earth: Russia, China and the "International Community" which is the American Empire.
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Behold! we are not bound for ever to the circles of the world, and beyond them is more than memory, Farewell!