Taiwan wasn’t always a pariah at the United Nations. For more than 20 years the Taipei-based government represented China there, including on the Security Council. That changed on Oct. 25, 1971, when the General Assembly voted to admit delegates from the People’s Republic of China. The Republic of China lost its seat and has been on the outside looking in ever since. Resolution 2758 marked the beginning of Beijing’s assault on Taiwan’s international status.

In 1970 Beijing’s supporters in the U.N. faced an uphill battle....

A Taiwanese flag during the national day celebration in Taipei, Oct. 10.

Photo: ANN WANG/REUTERS

Taiwan wasn’t always a pariah at the United Nations. For more than 20 years the Taipei-based government represented China there, including on the Security Council. That changed on Oct. 25, 1971, when the General Assembly voted to admit delegates from the People’s Republic of China. The Republic of China lost its seat and has been on the outside looking in ever since. Resolution 2758 marked the beginning of Beijing’s assault on Taiwan’s international status.

In 1970 Beijing’s supporters in the U.N. faced an uphill battle. Only a year later the proposal passed in a landslide. What changed? In July of that year, President Richard Nixon revealed that national security adviser Henry Kissinger had visited Beijing, that Nixon himself would do so before May 1972, and that the latter meeting would “seek the normalization of relations between the two countries.”

In 1971 there was a reasonable case for the Beijing government’s inclusion in the U.N. It had all the qualifications of statehood, and the 850 million people living on the mainland had no representation. It also seemed increasingly untenable to exclude a nuclear power.

There was no such case, however, for excluding Taiwan. At the time, it had diplomatic relations with 54 countries, including the U.S., Japan, Australia, South Korea, New Zealand, West Germany, Israel and South Africa. Its mutual defense treaty with the U.S. would remain in force for the rest of the decade. Taiwan, a founding member of the World Bank and the International Monetary Fund, remained a member of those organizations until losing its seats to China in 1980. A true reckoning would have recognized the existence of two states, each with a right to U.N. membership—a solution not unlike those later reached to admit both North and South Korea and both East and West Germany.

Instead, the U.N. has only compounded its 1971 error in recent decades. In the early 2000s, to explain why he had blocked a Taiwanese diplomat from speaking to the U.N. Correspondents Association, Secretary-General Kofi Annan invented a U.N. “one-China policy.” His successor, Ban Ki-moon, went even further, invoking Resolution 2758 and claiming that “the United Nations considers Taiwan for all purposes to be an integral part of the People’s Republic of China.”

But Resolution 2758 says nothing of the sort—it doesn’t even refer to Taiwan. The U.N. charter doesn’t give the U.N. the power to make territorial decisions, nor does international law recognize a U.N. vote as a legitimate means of drawing sovereign boundaries.

So what explains the U.N.’s legal and moral malfeasance in its treatment of democratic Taiwan—preventing it, for example, from even participating as an observer in World Health Organization meetings? The obvious answer is the U.N.’s deference to China. Beijing’s diplomats throw temper tantrums, mobilize partners to advance China’s interests, and have been especially successful at putting Chinese officials in positions of authority in various U.N. bodies such as the International Telecommunications Union.

The U.S. and its diplomatic allies have failed repeatedly to push back against China’s bullying. To mark Resolution 2758’s 50th anniversary, the U.S. should make clear to Beijing that absent far greater Chinese flexibility on Taiwan’s engagement in the U.N., Washington will launch a campaign to secure Taiwan’s full membership.

Such a campaign would feature American diplomats publicly making the case for Taiwan’s inclusion, introducing annual resolutions at the General Assembly, and coaxing recalcitrant U.N. members to support a seat for Taiwan. Faced with a fight over full membership for Taiwan, U.S. allies and partners might increase their own pressure on China to accept at least partial participation.

As the U.S. establishes the legal rationale for Taiwan’s participation—that it has all the attributes of a sovereign state under international law—Washington may have to rethink its own policy. When the U.S. formally recognized China, Washington acknowledged “the Chinese position that there is but one China and Taiwan is part of China,” and it has consistently stated that the resolution of the dispute must be peaceful. But a peaceful resolution is nowhere in sight as Beijing continues to increase military, economic and diplomatic pressure. And the people and government of Taiwan are no longer interested in unification with China.

A fight over Taiwan’s membership in the U.N. might at a minimum lead the U.N. to reverse its dubious reading of Resolution 2758 and in turn open the door for Taiwan’s greater participation in international organizations. But it could also move the U.S. and its allies to take a more principled position toward an island democracy that is increasingly important to the global order that the U.N. was intended to promote.

Mr. Schmitt is a senior fellow and Mr. Mazza a nonresident fellow in foreign and defense studies at the American Enterprise Institute.

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