Just two days after China sent a Shenyang J-11 fighter jet, a Xian H-6 bomber, and a Shaanxi KJ-500 on an “island encirclement” drill in Taiwanese waters, the U.S Air Force responded by flying two B-52 bombers and an MC-130J Commando II tanker near the Taiwan Strait sending a warning to Beijing, according to Taiwan’s Ministry of National Defense.

Pacific Air Forces Spokesperson Major Victoria Hight said the B52s flew along the east coast of the island while the other aircraft flew north to south over the strait. The defense ministry said in a statement that they were “closely monitored.”

The impasse between China and Taiwan goes back to the 1949 Chinese civil war. The defeated members of the Republic of China (ROC), led by Chiang Kai-shek, fled to the island, about 110 miles across the strait from the mainland.

Taiwan grew into a global economic player with close ties to the U.S. while China, under Communist rule, gradually assumed its current role as the second most powerful country in the world.

China continues to claim sovereignty over the island and has made it clear that it is willing to use force to achieve a total re-unification presumably under Chinese Communist rule. Chinese leader Xi Jinping described unification as “inevitable.”

Most Taiwanese citizens are opposed to reunification as evidenced by President Tsai Ing-wen’s reelection based largely on her promise that Taiwan will remain independent while still maintaining economic and trade ties with the mainland.

As for the U.S., it describes Taiwan’s status as “unsettled” acknowledging only that Beijing considers it to be part of China. Washington has no formal ties with Taiwan but is bound by law to help defend it due to H.R. 2479, the United States-Taiwan Relations Act that provides a framework for the U.S. to aid the self-governing island’s defense.

Sunday’s drill, according to a Chinese Defense Ministry statement, was a “combat-readiness patrol,” sending bombers, jet fighters and early warning aircraft flying through the Bashi Channel and Miyako Strait. Some destroyers and frigates were also involved in the exercise.

Another Defense Ministry statement issued on Tuesday said Monday’s exercises were joint naval and air drills in the waters off the southeastern coast of Taiwan, including air-to-ground assault and fire support exercises.

“The Chinese military’s combat-ready patrol is completely legitimate and is the necessary action taken to address the current security situation across the Taiwan Strait and safeguard China’s sovereignty,” the Chinese Defense Ministry statement said.

To make sure China’s message was heard loud and clear by Taiwan and the U.S., Chinese military expert Song Zhongping told the Global Times that the demonstrations “are not only warnings to Taiwan secessionists but also demonstrations of the Peoples Liberation Army’s (PLA’s) capabilities to solve the Taiwan question by force.” That message may have been the impetus for the U.S Air Force response on Wednesday.