Obama clearly wants to pull the U.S. out of Iraq and Afghanistan, which under Bush were big parts of the U.S. empire.

Clearly. And for a good reason, given how Afghanistan has always been resistant to external aggression and Iraq was Bush and Cheney's pet project, unrelated to 9/11.

Lots of Republicans think that Obama wants to greatly reduce U.S. military power

What do they think his motivation would be, other than possibly financial?

I take it you don't have much experience talking with leftwing college professors.

Some. The ex-hippie Berkeley types are rather annoying. Krugman is annoying. But to me any ideologically-motivated argument is annoying, because of its anti-rationality.

It's far from implausible to think that deep down Obama believes that U.S. military power has, with the exception of WWII, been a force for evil.

Eh, I don't see the connection. The leftwingers rarely hide their views. Obama has never expressed anything close to what you are describing and hasn't worked for any radical leftwing organizations (beyond a tenants' rights organization during his college years). He certainly supported left-leaning causes, like healthcare and welfare reforms, in the past, but he still does so, pretty openly. I grant you that his expressed views and actions have shifted rightward, and his actual views might be closer to what he held 15 years ago, but still solidly within the spectrum of DNC views. The odds of him considering the US military power being (a force for evil), given that he never expressed views like that, are pretty slim. Not that I personally approve of his policies and actions, the man has been a disappointment in terms of his competence level. But inept does not mean malicious.