China and Taiwan are getting along much better these days. Considering the stake the United States has in their relationship, that's good news.

But this is no time to let down our guard, as Chinese officials have been urging.

Wang Yi, who heads China's Taiwan Affairs Office, visited Washington last month to press the case against U.S. arms sales to Taiwan. At the same time, U.S. Undersecretary of defense Michele Flournoy was in Beijing for military talks, hearing the same pitch. The United States should not waver in the face of Chinese pressure to reduce the quality and quantity of American support for Taiwan. To do so would both jeopardize Taiwan's security and risk returning to a fractious, tense and dangerous cross-Strait relationship.

Clearly, the Chinese are testing the waters. One can only hope the Obama administration takes a firm stand. Taiwan's president , Ma Ying-jeou, has been lauded, and rightly so, for delivering tangible benefits from his engagement with China. His early moves have brought direct flights between the two, observer status for Taiwan at the 2009 World Health Assembly meeting, and myriad new economic opportunities. Such concessions by the Chinese are essential for President Ma. His credibility at home depends on delivering better relations with China.

But his reasons for pursuing "engagement"--to reduce tensions and offer new opportunities for Taiwan's many global interests--differ from those of the Chinese.

The people of Taiwan predominantly support the "status quo," essentially a de facto independence. Any Taiwan president straying too far from that position will rapidly lose domestic support. Ma therefore depends wholly on Chinese willingness to give Taiwan greater international breathing room. Yet, despite the recent ease in tensions, China continues to hedge on its Taiwan policy. Beijing feels a need to keep its options open--including the use of military force, something the mainland has refused to renounce. Beijing's substantial force modernization effort continues at full speed. Meanwhile, the People's Liberation Army remains focused on ensuring its ability to coerce Taiwan while deterring U.S. intervention.

Negotiating in these circumstances isn't easy for Taiwan. What strengthens its hand immeasurably is a robust and material U.S. security commitment. This backing underpins Ma's outreach and ensures a degree of Chinese respect for Taiwan's options. It gives him the strength to tell China "no." Entering any dialogue without the power to walk away is a losing proposition. The United States sells arms to Taiwan not to turn it into an offensive threat but in response to China's military threat. If power tilts too heavily in favor of China, as it does today, then America must assist Taiwan. Not doing so would create instability. It would also contradict U.S. law.

Given the thaw in the cross-Strait relationship, Ambassador Wang and his colleagues in Beijing no doubt presented a strong case for the United States to curtail support for Taiwan--particularly as the Obama administration has so many other issues on its docket for engagement with China. It would be tempting to simply let Taiwan and China "get on with it," doing less with Taiwan in the hopes that China will then be more receptive to supporting U.S. priorities such as climate change. Yielding to this temptation would be great folly.

China knows exactly what it wants in its relationship with Taiwan: unification. All of its decisions related to Taiwan are driven by that single goal, and it will look closely at ways to erode U.S. support for Taiwan, particularly in the area of defense. The Chinese continue to identify F-16s for Taiwan as a red-line issue, making the decision on that sale one that could have grave consequences for U.S.-China relations. In so doing, they seek to involve China directly in U.S. policy considerations of Taiwan's material defense needs.

If America chooses to back off--or simply take a minimalist approach--voters in Taiwan will notice. So will China. U.S. diffidence would erode Ma's domestic support for improved relations, as Taiwan's voters become nervous at the prospect of fewer options. China, by contrast, will become further emboldened by a softening of U.S. support for Taiwan. This would encourage fewer concessions and a heavier hand from Beijing. Surely, tensions would start to rise again. With reduced U.S. support, Taiwan will be in an even weaker position to deal with rising tensions. That could easily leave Taiwan and Washington with fewer--and far starker--policy options in the future.

Walter Lohman is director of Heritage's Asian Studies Center. Rupert Hammond-Chambers is president of the US-Taiwan Business Council.