Well, the Iraq war is interesting. The US sold a bunch of chemical/biological weapons to the Iraqis, and then years later used that as a pretext for war, being fairly sure that they would find them. Problem was, they weren't there any more, so the US looked stupid.

What happened to those weapons of terror? Well, the Kurds can probably tell you. Suffice it to say that not everyone was a big fan of Saddam, and some people were legitimately happy to see the US overthrow Saddam.

The main problems are this

1) bogus pretext for the war, lying to your own people

2) the means they used (bombing civilian infrastructure, cutting off power supplies to hospitals etc)

And on that note, the US has this insane idea that if they interfere with other countries, mess with their economics etc then it makes people upset with $local_dictator... but it doesn't work like that, it just makes them and everyone else in the world pissed off at the US.

3) What they did afterwards. Sticking around and trying to make the Iraqis pay for it all by looting their resources. Not good.

Afghanistan is even worse. The soviet union stuck their dick in the meat-grinder that is Afghanistan and were there for 20 years and it destroyed them and bankrupted them economically. How is the US going to fare any differently? Oh, you spent a trillion dollars on this war and your economy is in the hole for a trillion dollars. What a mysterious coincidence!