The USNS Impeccable was conducting an “ocean mapping mission” in the South China Sea when she was confronted by Chinese ships, who maneuvered close to her and attempted to snag her towed sonar array:

The exposure came as the American vessel USNS Impeccable was attempting to defend itself against what the Pentagon claimed was co-ordinated harassment and aggression from five Chinese ships. Being unarmed, the Impeccable turned its fire water hoses against two of the Chinese vessels that had come within 50 feet in a threatening posture.

Then, the Pentagon records in the admirably restrained language of international diplomacy, “the Chinese crew members disrobed to their underwear and continued closing to within 25 feet.”

(emphasis mine)

OK, it’s not naked aggression, it’s semi-naked aggression, but still…..

It should be noted that the ship was actually on an intelligence gathering mission:

The U.S. Navy’s description of the incident states that “a civilian crew mans the ship, which operates under the auspices of the Military Sealift Command.” Yet as one of five ocean surveillance ships, the USNS Impeccable (T-AGOS 23) has the important military mission of using its array of both passive and active low frequency sonar arrays to detect and track submarines. The USNS Impeccable works directly with the Navy’s fleets, and in 2007 operated with the three-carrier strike battle group in Valiant Shield 07 exercise in the Western Pacific.

USNS Impeccable is equipped with the Surveillance Towed Array Sensor System (SURTASS), a passive linear underwater surveillance array attached to a tow cable. SURTASS was developed as a floating submarine detection system for deep waters, and the Navy wants to add an active Low Frequency Array (LFA) to improve long-range detection of submarines in shallow waters.

Note that the ship was only about 75 miles from the Yulin naval base, where the Chinese have based their most advanced SSN, the Shang-class (Type-093), and that the Impeccable does carry the Surveillance Towed Array Sensor System (SURTASS).

I’m not sure if they recording acoustic signatures, or recording operations, or both, but this was not just an ocean mapping expedition.

The US government is claiming illegal harassment in international waters, and the Chinese government is claiming a violation of their sovereignty.

It’s probably a bit of both.