Vance Serchuk is a Council on Foreign Relations-Hitachi international affairs fellow, based in Tokyo at the Canon Institute for Global Studies. He writes a monthly column for The Post.

For decades, when U.S. policymakers contemplated conflict with China, their fears focused on Taiwan. Today, by contrast, Sino-American tensions seem to be on the rise everywhere but Taiwan, where relations between this island and the mainland have significantly improved.

Since Taiwanese President Ma Ying-jeou took office in 2008, the government in Taipei has inked 18 agreements with China, unleashing a surge of commerce and tourism. With trade barriers falling and direct flights proliferating — there are more than 600 a week, up from none five years ago — the Taiwan Strait is the only flash point in Asia where globalization and economic integration seem to be trumping historical antagonisms.

This island of 23 million stands out from the rest of the region in another respect: While the Obama administration has been deepening U.S. alliances across Asia, Taiwan has been conspicuous for its absence from that conversation. In a lengthy address this spring describing the future of the U.S. “rebalance” toward Asia, national security adviser Tom Donilon didn’t mention Taiwan once. Nor have other senior officials in major speeches about the region.

Judging by U.S. rhetoric, one might conclude that as the threat of cross-Strait conflict has receded, so has Taiwan’s strategic significance.

In fact, Taiwan is becoming more important for Asia’s future.

That is because, although ties between Taiwan and the mainland have improved since 2008, China’s relations with other neighbors have worsened — with Beijing exercising troublesome new assertiveness in the East China and South China seas, which are adjacent to Taiwan.

At the same time, China continues its double-digit growth in military spending — much of it devoted to weapons targeting the Pentagon’s ability to project power in the western Pacific.

All of this poses a challenge to the once-popular notion that, if only the threat of conflict over Taiwan could be disposed of, U.S.-China relations would be assured a stable, peaceful footing. Instead, the experience of the past five years has fanned fears in Asia that China’s ambitions extend beyond Taipei to include displacing the United States as the dominant power in the region.

And that takes us back to Taiwan. If there is a serious prospect of an extended security competition with China over the skies and waters of the western Pacific, Taipei will be pivotal in shaping how it unfolds.

Partly this is the consequence of geography. Taiwan is smack in the middle of the “first island chain” — the string of islands between Beijing’s growing fleet of ships and the deep waters of the Pacific. Chinese military strategists have long recognized that control of Taiwan is critical for breaking through this barrier and projecting power beyond.

That is why the thought of Taiwan ever falling back under Beijing’s control is so alarming for defense planners in neighboring countries. Tetsuo Kotani, one of Tokyo’s top maritime security experts, told me that “losing Taiwan to [China’s military] would be a game-changer for Japan and the regional naval balance.”

Likewise, Toshi Yoshihara and James Holmes, professors at the U.S. Naval War College, describe Taiwan in their book “Red Star Over the Pacific” as “the one geographic asset that can grant Chinese forces direct access to the Pacific” and note that “a nation in possession of Taiwan has the freedom to cut sea communications from Northeast to Southeast Asia.”

Beyond geography, Taiwan is also a litmus test of U.S. reliability. If Washington were to be perceived as abandoning the island, a vibrant democracy, it would provoke a wider crisis of confidence among those in Asia counting on the United States for protection from China.

None of this is to say that reduced tensions between Taiwan and the mainland shouldn’t be welcomed. To its credit, the Obama administration has authorized nearly $20 billion in arms sales, granting Taiwan inclusion in a visa waiver program and resurrecting talks for a trade and investment framework agreement.

Despite these measures, the balance of power between China and Taiwan continues to trend in Beijing’s favor, warranting a more ambitious U.S. security and economic agenda with Taipei.

This could include not only additional arms sales — in particular, submarines Taipei has long sought — but also quieter steps such as intensified training and exercises with the Taiwanese military, permitting U.S. general officers to visit the island, deeper cooperation on cybersecurity and a push for a free-trade agreement with Taipei or its incorporation into the Trans-Pacific Partnership.

What should also be public and unambiguous is the United States’ continuing national interest in Taiwan’s fate. The Obama administration’s near-silence on this while it trumpets the broader pivot to Asia sends exactly the wrong message.

Taiwan has become one of Asia’s bright spots. That shouldn’t make it an American blind spot.