I feel that you are severely mistaken in that belief. My reading of US history leads me to the opposite conclusion.
The USA is a serious contender for the most aggressive nation in the history of the planet. In the entire span of the USA, there has not been a single decade in which the US military did not attack.
The longest span of peace ever tolerated by the USA was seven years: the period just before WWII, spent flirting with fascism and selling a shitload of guns.
Stupid, destructive and counterproductive invasions of weaker nations is what the US military does. They're the world leaders at it.
Just about everyone, at the moment.
To their west, they have the Saudis. To their east, the Pakistanis . North, the Russians...and from the sea, Americans.
Try to see it from their point of view. Your country spends a few centuries getting fucked by foreign imperialists  then just as you finally kick out the quislings and get a decent government, a new imperialist arrives. Your government is brutally overthrown by foreign intervention, and a murderous dictatorship takes its place.
When you finally manage to overthrow that bastard, you're attacked by your neighbour in a war that makes the Somme look mild. Horrendous sacrifice by your people halts the invasion and turns the tide, but the counterattack is halted by the renewed intervention of foreign powers, both local and imperial . Their interference causes this unimaginably brutal war to drag on for eight years.
A decade later, due to an event that had nothing to do with you, the imperialist that fucked you before declares you to be a member of a three-part "Axis of Evil" and immediately annihilates one of your fellow accused (who also had nothing to do with the supposed justification). Crippling economic sanctions are imposed upon your country that severely limit the ability to modernise your military.
The other surviving target in the "Axis" responds by displaying nuclear weapons. Although they're still subject to constant rhetorical threats, their nuclear deterrence appears effective.
The foreign power then massively arms the most hostile of your neighbours, vigorously encourages them to aggression and specifically points them at your country. While apparently forming an alliance with the largest of your other historical oppressors.
If I was an Iranian, I would want nukes as fast as I could possibly build them.
 Pre-US invasion, the Afghan conflict was an Iranian-Pakistani proxy war.
 Russians as much as the Brits.
 US and Saudis primarily, but the French and Brits got in a bit as well.