Tensions between two nuclear-armed powers, China and India, are rising.

At contention is a mountainous border area between China, Bhutan, and India.

Alleging that Indian soldiers are threatening Chinese territory, on Monday, the Chinese military threatened India to think carefully about its continued deployments. According to Reuters, a Chinese spokesman remarked that "Shaking a mountain is easy, but shaking the People's Liberation Army is hard." The spokesman continued, "India should not leave things to luck and not harbor any unrealistic illusions."

These are Chinese fighting words. They are also totally unjustified.

For a start, China has been far more aggressive in the contested border regions in question. Openly building railway lines and other symbols of state power in disputed areas, China has given a one-fingered salute to India's Hindu-nationalist government. And while China's motivations are partly trade-related, their central concern is one of pride.

China knows that India's growing economy, democratic foundation, and increasingly close relationship with the United States make it a major world power. Crucially, China wants to ensure the new kid on the block doesn't disrupt its efforts to dominate Asia.

For that reason, America should offer clear support to India as it confronts China. As I've noted, India-U.S. relations offer special value in our shared ability to deter and counter growing Chinese aggression. But with China now threatening U.S. military personnel elsewhere, we should also recognize that the stakes are growing. When one considers the nuclear dimensions at play, the risks are great. Correspondingly, both India and the U.S. are much stronger standing together. Only by doing so can we deter Chinese imperialism.

Fortunately, India isn't bowing to Chinese threats. On the contrary, the Indian news outlet Zee News reports that "Indian soldiers deployed in the disputed area have pitched in tents, in an indication that they are unlikely to retreat unless there was reciprocity from China's PLA personnel ... A steady line of supplies is being maintained for the soldiers at the site, official sources said, signaling that Indian Army is not going to wilt under any pressure from China."

This is good news because China revels in its belief that its vast military can overwhelm any adversary. Yet it also knows India would be no pushover. In the event of a conflict, China might suffer major military and economic losses. By putting faith in its own power, India reinforces its deterrent posture in refusing to accept Chinese blackmail.

The U.S. should not sit idle here. We must support our ally and rally the world in diplomatic condemnation of China's threats. Moreover, going forwards, as I've argued before, President Trump should work hard to strengthen his relationship with Prime Minister Narendra Modi. On issues of nuclear security, trade, and international order, Modi offers a partnership for 21st-century stability. The alternative is a world ruled by Chinese imperial aggression.

And we know from 1930s imperial Japan that the choice of appeasement is the surest path to eventual war.