More than 70 years ago the United States redesigned what an alliance could be used for. For centuries, countries allied with each other in order to fight and win specific wars. But after the devastation of World War II and the beginning of the Cold War, the US needed help preventing another from starting.

In the span of just a few years, it joined a collective alliance with 11 other countries, called the North Atlantic Treaty Organization, and signed several alliances with countries in Asia. These agreements obliged the US to protect these countries — but it also gave them an unprecedented number of partners around the world.

This system largely proved to be a success. A third world war never happened, and the US won the Cold War. But in the past decade, the US and its allies have been drifting apart, and many believe the system needs an update. The question is what to do about them.

Watch the video above to learn about the US alliance system and what the 2020 Presidential candidates plan to do about it.

This video is the sixth in our series on the 2020 election. We aren’t covering the horse race; instead, we want to explain the stakes of the election through the issues that matter the most to you. To do that, we want to know what you think the US presidential candidates should be talking about. Tell us here: http://vox.com/ElectionVideos.

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