The first confrontation between the US and Chinese navies concluded on Tuesday, when the Chinese returned an underwater oceanographic drone captured in international waters outside of the Philippines.

“The incident was inconsistent with both international law and standards of professionalism for conduct between navies at sea,” said Peter Cook, a Defense Department spokesman.

And President-elect Donald Trump tweeted of the incident, “Let them keep it.”

The drone, composed of about $150,000 in off-the-shelf technology, ranks low on the list of US-China military confrontations — as past confrontations have involved air crews being detained and planes grounded — but it marks an important shift in what is quickly becoming one of the most militarized and fraught regions on the planet.