China's PLA military has long been known to intimidate and threaten the self-ruled island of Taiwan, mainly with large military exercises which sometime encircle it partially or completely, or else with a heavy naval boat presence in the Strait of Hormuz.

Taiwan's military often reacts by scrambling its own fighter jets to closely monitor the PLA maneuvers - seen as a natural defensive and reactive move. But this week, in a rare moment, Taiwan is finally doing some proactive flexing of its own.

"Taiwan fired U.S. mobile missile launchers into the strategic waters directly facing China for the first time, sending a message of resolve to Beijing and Washington," The Wall Street Journal reports.

This involved over 30 test rocket launches via truck-mounted High Mobility Artillery Rocket Systems, or HIMARS. Importantly, the launch site was an area near a river mouth on Taiwan's western coast.

"This is sending a message to the Chinese that they are going to get hit hard if they try to come across the strait—and will end up with far fewer ships than they started with," Grant Newsham, a retired US Marine colonel who served in several Indo-Pacific roles, told WSJ.

The WSJ continues with further context:

The drill was the highlight of two days of military exercises showcasing Taiwan’s preparations to combat an amphibious invasion. China considers democratically self-ruled Taiwan as part of its territory and hasn’t ruled out potentially using force to absorb the island.

...Such exercises also serve as a signal to Washington that Taiwan is committed to defending itself and deserves U.S. support, with a $14 billion U.S. arms package currently on hold.

Back in April, Chinese President Xi Jinping addressed cross-strait relations: "All sons and daughters of China share the same Chinese roots and the same Chinese spirit. This originates from blood ties and is deeply embedded in our history – it cannot be forgotten and cannot be erased," he said at the time.

Officially, Biejing seeks a 'peaceful reunification' of the independent island to the mainland, while many Washington officials fear it could at any point launch an outright invasion and political takeover.

But the reality remains that any PLA direct military intervention would likely be in response to a provocation, and wouldn't just materialize out of thin air. Beijing has at times warned that the growing billions of dollars in arms that Washington has been providing Taiwan could be just such a provocation. 

Xi's China has also been very alarmed at the growing (direct) US military footprint in its own backyard, given the presence of American military advisers, said to be present in some of Taiwan's small islands which lie close to the Chinese mainland.