Donald Trump has said many times that the U.S. should withdraw from the nuclear agreement with Iran. Other leaders don't agree. They point out that Iran has abided by the agreement, and inspections have shown they are not currently working on getting a nuclear weapon. Macron (France) and Merkel (Germany) recently came to this country to try and talk Trump out of withdrawing from the agreement, and they left believing they had failed.
It now looks like that Trump will actually withdraw the U.S. from that agreement. Iran has said if he does, then they will consider the agreement to be null and void. They could well resume their nuclear weapons program.
That's not the only bad thing that could come from withdrawal. It would also show the world that Trump has no respect for agreements with other countries -- and this (along with withdrawal from the Paris Accords on global climate change) would discourage other countries from entering into an agreement with the United States. And with the North Korean talks coming up soon, it would show Kim Jong-Un that Trump cannot be trusted (and destroy the slim likelihood that those talks could be productive).
Then we have the wishes of the American public. Trump may think he has convinced Americans that we should withdraw, but that is simply not true. A new poll shows support for withdrawing is very weak. Only 25% of the public (and only 30% of registered voters) support withdrawing from the Iran nuclear agreement. And all groups have far less than majority support from withdrawing. Support is highest among Republicans, but it is only 48% even there.
The chart reflects results of a new Economist / YouGov Poll -- done between April 29th and May 1st of a random national sample of 1,500 adults (including 1,273 registered voters). The margin of error for adults is 3.1 points, and for registered voters is 2.9 points.