As China's industrial importance and strength has grown it has been pushing it's military power. More so than in the past.
And China is well aware of Japan's Article 9 of the Japanese Constitution.
"Article 9 of the Japanese Constitution is a clause in the National Constitution of Japan outlawing war as a means to settle international disputes involving the state. The Constitution came into effect on May 3, 1947, following World War II. In its text, the state formally renounces the sovereign right of belligerency and aims at an international peace based on justice and order.""The article also states that, to accomplish these aims, armed forces with war potential will not be maintained, although Japan maintains de facto armed forces, referred to as the Japan Self-Defense Forces..."U.S. Warns China Over East China Sea Maritime Grab"In the South China Sea, China has triggered maritime disputes with Vietnam, Philippines and several other states by declaring that 90 percent of the sea is Chinese territory. An air defense zone over the South China Sea could be a next step.""In the South China Sea, China has triggered maritime disputes with Vietnam, Philippines and several other states by declaring that 90 percent of the sea is Chinese territory. An air defense zone over the South China Sea could be a next step."The area in red is what they are now claiming. For reference, the Japanese island to the right of the word "Japan" is over 200 miles (320+ kilometers) long.
China is the 800 pound gorilla in the room. You may not like what it's doing, but you don't want to provoke it into action.
