Why did the Americans start selling weapons to nations at war?

The US transfers to these countries are meant to ensure allies security, as arms transfers from the US brings with it security guarantees, which basically entail diplomatic and military assistance in case of troubles

Explanation:

During the Cold War, American arms sales became part of a broader strategy to deter the Soviet Union from invading Western Europe. In the wake of the 9/11 attacks, the United States sold weapons to Afghanistan and Iraq to bolster their ability to defeat the Taliban, al Qaeda, and the Islamic State.