The following does not differ, in substance, from your argument above, but it does perhaps give a slightly different perspective on that substance. By "the Empire" I presume you mean, primarily, The U.S. Empire, and certainly the U.S. continues to act (and will continue to do so) _as if_ it wasits own Empire that was at stake.
But let's shift the focus a bit. "Empire" in reference to capitalist empires has always been something of a metaphor. The British Empire _hurt_ the English nation as a whole, benefitting only a _sector_ of British capital, not the whole even of that. The same has been true of the "American Empire." Ellen Meiksins Wood has it right I think in the title of her her work: The Empire of Capital. (That also is much better than "globalism," which enocourages daydreams such as those in Hardt & Hegri.) Capitalism can only flourish inside a state; it requires state support. But though capitalism has now penetrated the whole world, and different national capitals are intertwined, a World State is out of the question.
[As is occasionally noted, Kautsky (though in a perverse way) has also suddenly become 'relevant' for us. I say perverse way because of his assumption that a "global capitalism" would lead to peace, while in fact it leads, in Wood's term, to endless war. More on that below.]
Therefore individual states of the most diverse forms of governnment and internal balance of forces must be depended on to keep the "world" safe for business. THAT requires an international Cop - and much to the satisfaction, actually, whatever their occasional rhetoric to the contrary, ALL the first and second line capitalist states* have been very happy to leave that task to the United States. (*The EU and Japan; Russia and China; Brazil and India.) Several times not too long ago, usually in response to the list's Pollyanna, I emphasized that it was important to honor the tautology that capitalism is capitalism: that it is, in fact, disastrous to see the capitalism of Russia or China or Brazil as constituting any kind of counterweidght to or improvement over U.S. capital; in other words, the Empire of Capital offers _more_, not less horror, than the American Empire.
And it still _seems_ to be "The American Empire" for a number of reasons, _chief_ of which is,as mentione, that the U.S. is acting as The Global Cop - and doing so with the complete and _willing_ acquiescence of other capitalist powers (e.g., the EU and Japan). And of course certain sectors of U.S. capital, for example energy and banking, do gain greatly from the U.S. role of Global Cop. Nevertheless, that role is primarily performed for global capitalism, for Mexican and Indian and Brazillian capital as well as for U.S. capital.
I believe the Iraq War (as much as it played to the interests of a few sectors of U.S. capital) was always primarily an instance of maintaining "order" in the world for Capital, of punishing (as an example) a state that was seen as setting a bad example. Afghanistan's geopolitical importance to world capital is obvious, hence the satisfaction, it seems to me, with which China and Russia approve of coalition activities there. Better the U.S. and Nato than them, they must be thinking, to perform this necessary task.
And Israel remains a key geopolitical and military resource for the carrying out of this endless struggle to maintain a world safe for capitalism. A democratic Palestinian state, a _real_ state, could infect the whole of the Arabic world, whose authoritarian states are of such crucial importance to the Empire of Capital. Revolution in Egypt, in Saudie Arabia, in Pakistan is unthinkable! And thus Israel must be protected at whatever cost. If you draw a trajectory from Israeli-U.S. policy over the last 40 years it has only one outcome, but one tha t must be carefully disguised until after the fact: The Palestinians must be ejected from their land, drive out by terror and economic misery, scattered about the rest of the Arabic world under the thumb of those ruthless authoritarian regimes.
That's very rough and ready. But it does make sense of U.S. policy without positing an "American Empire" the precise content of which is difficult to describe.
Carrol