We started being an empire the moment "compelling state interest" became such a tool of the state, a way to overrun the Constitution and trample on individual rights.  We went farther down the road when "public safety" became a mantra that nearly equated with "compelling state interest" (anyone who recalls the old "committee for Public Safety" should have shivered at the mere invocation of the term).
The military adventurism is a sign of an executive who is at heart an emperor, just as is the flagrant use and abuse of executive orders.  Reagan was an emperor at heart; so was Clinton -- but Bush beats the two of them combined, in spades.
Truly observant people have noted and warned that we are becoming an empire as early as the mid-60s.  Empires can come about in several ways; we are following the Roman model, where empire takes over to perpetuate the corpse of Republic, which failed due to those in leadership putting their own interests first.
I see a seed of empire in FDR's redefining the American Dream from liberty to prosperity:  we seek riches, not rights, and when we do seek rights we do so selfishly, without regard for everyone else.  When that defines the masses, it defines the politicians moreso, since they arise from the masses and distill the aspirations that call to us all.
My grandmother, according to her diary, understood the duty to vote as the duty to choose what is best for the Republic, and that belief was seemingly universal.  But FDR appealed to our selfishness, and now we vote "in our interests".  Goldwater had it right; out true interest is liberty -- but the American public rejected him, and chose the imperial-minded JFK... and the even more imperial LBJ, who set the tone for those who followed.
So we get a Congress that mirrors the Roman Senate so eerily to be frightening, along with organizations fighting to retain Republican (I don't mean the party, but the philosophy) virtues, ranging from the NRA to the ACLU.  Yet the latter have become imperial in thought, and therein lies the deathknell of Republic, and the transition of the public soul to empire.
America is becoming an empire because we have abandoned our founding principles of liberty, and instead embraced power.  Isolationism might have saved us, for it would have kept us from believing that because we 'saved the world' we had the duty -- and the right -- to guide it.  But the isolationist impulse of today, the emotion-driven desire to go it alone, and the conviction that we can do so, is a disease, as are all apparent Republican virtues driven by imperial desires.
Becoming empire may ensure survival for a number of generations more, for while we cannot conquer any foe, we can crush them.  Yet empire will bring a more spectacular end -- one that could be disastrous for the whole globe.
Cheney and his cohorts, and the crowds cheering for McCain, see empire as glory, as advancement.  In truth, it is a sign of rot.