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The United States sells weapons around the world. It sells them to governments it approves of, such as the United Kingdom, Canada, the Netherlands and others.

But it also sells to countries that - to put it in the best light - it has mixed feelings about: Saudi Arabia, Egypt, Pakistan, Iraq, Rwanda to name just a few. Israel presents its own unique challenges, being both a close military ally and a democracy, but with policies that drive Washington mad.

The decision on what countries get access to which weapons from U.S. manufacturers is made largely by the Pentagon, arms expert William Hartung explains, in close consultation with the industry. The consultation is so close, in fact, that the Pentagon often acts as a broker, helping to put deals together.

Congress rarely gets involved. The current discussion among lawmakers over whether to sell new weapons to Saudi Arabia after alleged human rights abuses in Yemen is extremely unusual and experts believe the sale will still go through.

Arms sales are about keeping foreign governments sweet and keeping other nations’ manufacturers out. And once a country decides to go with U.S. weapons, they’ll need ammunition, spare parts and repair services, forging a relationship that lasts for years.

But how does the United States make sure that the weapons it sells don’t get resold, or fall into the hands U.S. enemies? And what role does the black market play? Listen to this week’s episode of War College to get the answers.