Whether or not it’s prudent for the United States to continue its war with Iran, a new constraint is becoming public knowledge: the depletion of crucial U.S. munitions. According to the Center for Strategic and International Studies (CSIS), the United States has used up as much as half of certain key defensive and offensive missiles in this year’s conflict with Iran. The running down of these munitions diminishes the White House’s ability to project power and emboldens its adversaries.

On the latest episode of FP Live, I spoke with Tom Karako, the director of CSIS’s Missile Defense Project, to understand the extent to which the United States has expended its inventory of missiles, and what that means for its foreign policy. Watch the full discussion on the video box atop this page, or try the FP Live podcast on Apple, Spotify, or YouTube. What follows here is an excerpted transcript of our discussion.

Ravi Agrawal: So we’re going to discuss several different types of munitions. Some of this is going to get wonky, but I just want to start with the one that seems to be the most well-known: the Patriot missile interceptors. Just very briefly, can you explain how they work in layman’s terms and why they seem to be so popular?