Iran has abided by the terms of the Obama-era agreement since the U.S. withdrew last year. But Tehran said on Monday that it would exceed the limits on uranium stockpiles within days unless European countries helped it circumvent American sanctions. Here’s a look at the background of the dispute.

The details: The additional troops come after 1,500 were deployed in May. They will be used mainly for surveillance and protecting American forces already in the region.

News analysis: Mr. Trump came into office vowing to solve nuclear crises in both Iran and North Korea. He is now confronted with a challenge that’s familiar to his predecessors, our national security correspondent writes.

Related: President Xi Jinping of China plans to make his first state visit to North Korea this week. Several analysts said they expected him to try to revive the failed disarmament talks between Pyongyang and Washington before he meets with Mr. Trump next week.