The United States will pull out of the Paris Agreement on combating climate change, President Donald Trump announced Thursday, a decision that makes good on one of his key campaign pledges but deals a devastating setback to international efforts to curb global warming.

Im fighting every day for the great people of this country, Trump said at a press conference in the White House Rose Garden. Therefore, in order to fulfill my solemn duty to protect America and its citizens, the United States will withdraw from the Paris climate accord.

Trump said he was willing to immediately work with Democratic leaders either to negotiate the U.S. back into Paris or devise a new pact.

If the obstructionists want to get together with me, lets make them not obstructionists, Trump said. Well sit down with the Democrats and all of the people who represent either the Paris accord or something thats much better than the Paris accord, and I think the people of the country will be thrilled.

Trumps announcement ends months of suspense characterized in recent weeks by a reality TV-style cliffhanger over what he would do, as some of his White House advisers urged him to keep the U.S. in the deal.

With his withdrawal decision, the U.S. joins Syria and Nicaragua as the only countries outside the agreement to combat climate change. Other countries, led by the European Union, China and India, pledged to forge ahead in the effort without the U.S. But the loss of the worlds second-largest emitter of greenhouse gases will have a traumatic effect on the fight against global warming.

The move is a particularly egregious repudiation of the international community because the Trump administration could have negotiated for lower emissions targets under the Paris Agreement, officials and the pacts advocates said. Because of that, the diplomatic fallout will likely be harsher than when President George W. Bush rejected the 1997 Kyoto climate agreement.

When that administration refused to implement the deal in 2001, other nations knew that the U.S. disapproved of giving developing countries a pass on emissions, and the reduction target was more aggressive. But the terms of the Paris Agreement were brokered to meet U.S. demands.

The reaction will be substantially worse because when we rejected Kyoto, other countries understood why, Susan Biniaz, the State Departments longtime former lawyer on climate change issues, told HuffPost Thursday. But in case of Paris, its inexplicable why we would be leaving. We negotiated it largely to U.S. specifications and to fix the Kyoto problems.