April 10 marks the 40th anniversary of the US' Taiwan Relations Act (TRA), which both the US and the island of Taiwan place great emphasis on.

This act, a US domestic law, took effect a few months after China and the US established diplomatic relations in 1979. The act has served as the "legal basis" for the US to interfere in China's internal affairs and make moves in the Taiwan Straits, including arms sales to the island.

"Taiwan independence" forces have regarded the TRA as their spiritual prop and also a stabilizer of ties between the island and the US.

The act is actually a sign of the US' declining ability to contain the Chinese mainland. Taiwan was once a public US ally. In the late Cold War, however, the US had to turn to Chinese mainland as its ability to contain the Soviet Union was not equal to its ambition. The TRA provided pivot for the US' retreat.

Superficially, the TRA is taken seriously by Washington. The US even enacted the Taiwan Travel Act and the Asia Reassurance Initiative Act of 2018 on the basis of the TRA to interfere in China. But, overall, the TRA's actual impact on the situation in the Taiwan Straits is declining. The US has not only lost its dominance in the Straits, but also faces an irreversible decline in its coercive power politically and militarily in this region.

An act can only be effective with strong implementation power. In 1979, the US enjoyed overwhelming military and economic advantages in the Western Pacific region. But now, the Chinese mainland's strength in the Straits and its surrounding areas has grown extraordinarily. The TRA has historically lost its power.

The mainland was once not powerful enough to project its influence on the island. It was Taiwan's military strength and the TRA that emboldened "Taiwan independence" forces. At that time, the Chinese mainland defense of the one-China principle mainly depended on the US' adherence to the three joint communiqués between China and the US. Even the tiniest move of the US was given importance.

The Chinese central government passed the Anti-Secession Law in 2005. The mainland's growing comprehensive strength and military power have provided more ability to enforce this law. Today's mainland has increasing capabilities to ensure that the one-China principle is widely recognized.

When China-US relations were better, the US placed particular emphasis on the three joint communiqués. Now it stresses the TRA more often. The TRA is bound to collapse if the US uses it to confront the China's Anti-Secession Law.

Should Taiwan authorities believe the TRA's vague commitment of protecting the island, or should they fear the Anti-Secession Law's warning of employing non-peaceful means to suppress "Taiwan independence?" The latter's deterrence is obviously more credible.

The future of the cross-Straits situation does not depend on Washington's explanation of the TRA, or on how much it will enhance its relations with the island. It depends on how fast the mainland's attraction for Taiwan grows, and on support of the one-China principle provided by the mainland's increasing military strength.

People's Liberation Army fighter jets crossed the "middle-line" of the Taiwan Straits recently, and this is only a warning. The mainland can take further military action much more powerful than the political show performed by the US and the island. The two should view the TRA realistically, or else they will be dragged into an unaffordable cross-Straits situation.