In 2015 the United States and several other nations made a deal with Iran that offered the Iranian government $150 billion and an end to economic sanctions in return for which Iran pledged to postpone its nuclear weapons program for ten years.
It was a terrible deal for a host of reasons, not the least of which is that President Obama didn't even submit it to the Senate for ratification but made the agreement entirely on his own. President Trump has taken some heat for letting the agreement lapse on May 12th, but he really couldn't have done otherwise.
As Israeli Prime Minister Benjamin Netanyahu reaffirmed last week, the Iranians, the world's worst terrorist state, have been lying all along when they've denied having nuclear weapons aspirations. In any case, the agreement would not prevent them from restarting their program in 2025 and developing the capability of mounting nuclear warheads on missiles within a few years or even months.
In other words, the Iran deal allowed the world to sleepwalk toward Armageddon, deluding itself into thinking that peace had been purchased, as if Islamic fanatics could be bought off with money. In fact, if Iran is allowed to continue its nuclear weapons program Saudi Arabia, Egypt, and Jordan will all insist on building their own nukes, and it's almost inevitable that those weapons would be used.
Back when the deal was made with Iran Dennis Prager put together a video which explains in just a few minutes why this was such a terrible agreement. It's worth watching so as to better understand why President Trump chose to take the step that he did.