President Trump said on Thursday that the United States was officially withdrawing from the Paris climate agreement, following through on a campaign promise and defying fellow world leaders who had pleaded with him to remain a party to the pact.

The move was sure to provoke a sharp backlash from the rest of the world, and it could prove to be a major setback for international efforts to avert drastic global warming.

The world’s nations had already been struggling to reduce emissions deeply enough to prevent global average temperatures from rising more than 2 degrees Celsius (3.6 degrees Fahrenheit) above preindustrial levels, the threshold deemed unacceptably risky. The Paris climate deal was meant to provide a structure to ratchet up those efforts over time. Now, the world’s largest economy has clearly stated that it wants no part of that process.

The big question now is how other countries will respond to Mr. Trump’s withdrawal: Do they keep pressing ahead with climate action anyway, or does the accord start to unravel without the United States? Here’s a look at what happens next.