The Chinese government has vowed to return a U.S. underwater survey drone that was seized on Thursday in the South China Sea, effectively ending a brief standoff between the two nations. “Through direct engagement with Chinese authorities, we have secured an understanding that the Chinese will return the UUV to the United States,” Pentagon spokesman Peter Cook said in a statement. The statement came mere hours after China’s Defense Ministry had said the drone would be returned but criticized the United States, saying that the way it “hyped the case in public” wasn’t helpful for a speedy resolution.

So, what was President Elect Donald Trump’s response? He doubled down on his initial statement in which he called the seizure an “unprecedented” theft, saying that since Beijing stole the device it’s now theirs. “We should tell China that we don’t want the drone they stole back – let them keep it!” Trump wrote.

China steals United States Navy research drone in international waters - rips it out of water and takes it to China in unprecedented act. — Donald J. Trump (@realDonaldTrump) December 17, 2016

We should tell China that we don't want the drone they stole back.- let them keep it! — Donald J. Trump (@realDonaldTrump) December 18, 2016

China said it seized what was an unknown device to prevent damage to its ships. “In order to prevent this device from posing a danger to the safe navigation of passing ships and personnel, the Chinese lifeboat adopted a professional and responsible attitude in investigating and verifying the device,” Chinese Defense Ministry spokesman Yang Yujun said.

China has long complained about U.S. surveillance operations in the South China Sea and analysts say there could be several reasons behind the seizure. The Wall Street Journal summarizes the possibilities:

Experts said the incident could be an attempt to widen a wedge between the U.S. and the Philippines, a U.S. ally, or to test the resolve of U.S. President-elect Donald Trump after he suggested he would challenge China on trade and territorial issues. Beijing could also be trying to exploit inertia in Washington in the waning days of the Obama administration.

Whatever the case, the seizure amounted to “one of the most brazen actions that the PLA Navy has taken against U.S. Navy for a very long time,” Ashley Townshend, a research fellow at the U.S. studies center at the University of Sydney, tells Bloomberg. “Against a background of rising tensions in the South China Sea and Trump’s increasingly hawkish comments on China policy, this incident will be a serious test for U.S.-China relations.”

A piece in Global Times, a newspaper affiliated with the Communist Party, cites Chinese military analysts saying the drone should be a wake-up call that the U.S. must stop “its spying activities in the South China Sea.” China has captured underwater drones before but “the one we seized on Thursday is new and more advanced than before and might carry valuable information,” an analyst said.