Why the US sent a missile destroyer into Chinese-claimed waters
"...The goal, according to the US, was to challenge China's claims over what are generally understood to be international waters. China was furious. A spokesperson said the Lassen "illegally entered" the waters and "threatened China’s sovereignty and security interests." On Tuesday, China sent two ships — the missile destroyer Lanzhou and patrol boat Taizhou — to the area and told the American ship to get out. The Americans ignored them..."
Reference: www.vox.com .