Could it be that these wars exposed how primitive guerrilla forces can bleed the mighty American military dry? It's not like the world has seen a very convincing show of force.
I for one worry that the bluff has been called and that every dictator in the world now knows that apart from the threat of nuclear war, the American military's capabilities are far more limited than some perhaps had thought. When relatively poor and simple battlegrounds such as Iraq or Afghanistan cause trouble one or two decades on, then what could America do against more advanced enemies or against more enemies?
This is to me the biggest worry: occupying two such relatively simple countries is a challenge. The Russians, the Chinese, Iran, heck even the tiny Syrian government, can now see that America had lots of trouble with what is currently nation No 54 and nation No 111 measured by GDP. Yes, I know that GDP does not equal military strength and that the US was officially not fighting the Afghan government. But in reality, the US was fighting primitive guerrilla forces in a primitive country. And the US didn't really come out impressing anyone with their strength.