World leaders struck a compromise on Saturday to move forward collectively on climate change without the United States, declaring the Paris accord “irreversible” while acknowledging President Trump’s decision to withdraw from the agreement. Differences between the United States and other nations on climate, trade and migration made for a tricky summit meeting, which unfolded amid large protests that sometimes turned violent, with several injured and demonstrators setting fire to cars and looting in the streets of the German city. Trump tends to suck all the media air out of a room, even in Germany, where he is deeply unpopular. This summit meeting was always going to be primarily about Mr. Trump and his first meeting with President Vladimir Putin of Russia.