BRUSSELS (Reuters) - U.S. President Donald Trump muscled himself to the front of the world stage in Brussels on Thursday, firmly pushing aside the leader of soon-to-be member Montenegro at a NATO summit. Footage showing the leaders caught Trump pushing his way past Prime Minister Dusko Markovic, then confidently adjusting his suit as he emerged in the front of the group, closer to NATO head Jens Stoltenberg. Video of the incident went viral and came as Trump delivered a tough message to NATO allies to dramatically increase their defense spending to ease financial burden to the United States. White House spokesman Sean Spicer, asked about Trump moving past Markovic, said he had not seen the video but assumed the U.S. president was moving to his designated spot. The NATO mission of Montenegro, which is joining the Western military alliance against stiff opposition from Russia, did not immediately respond to a request for comment. The small Balkan country will formally become NATO’s 29th state in June but Markovic joined the Brussels summit, Trump’s first, and a ceremonial opening of the alliance’s new headquarters. During the leaders’ joint photo shoot in the new building, Canadian Prime Minister Justin Trudeau encouraged his counterparts to wave for the cameras. Some did as Trudeau requested, but Trump did not raise his arm. Trump smiled briefly, but otherwise kept a serious expression on his face for the brief session.    