For centuries, the relationship between China and India was the diplomatic Dog that Didn’t Bark. The two largest, most populous, most durable Asian countries, for most of their collective history, have lived alongside each other with an almost studied indifference to the military, economic and cultural activities of the other. This dynamic began to change in the postcolonial period, but slowly, unevenly and with as much backtracking as forward progress. However, the recent news that Delhi and Beijing may be establishing a military hotline – reminiscent of the admittedly apocryphal “red telephone” between the White House and the Kremlin – has shown how much the Sino-Indian relationship has expanded and matured in recent years – and also how much distance still remains. How India and China manage their relationship will have global consequences. Their sheer size influences global markets in commodities, and China’s stock market gyrations have already begun to have knock-on effects around the world. And as two nuclear-armed states with long-term unfinished territorial business between them and a good amount of mutual suspicion, diplomatic missteps between India and China risk nuclear escalation. For most of their history, geography was the primary reason that the two countries maintained a diplomatic distance, keeping their interests separate and avoiding substantial political and economic exchanges. Then, as the modern era dawned, China descended into domestic chaos and India found itself a direct colony of Britain, precluding any deeper ties as long as those conditions persisted. Only in the early 1950s did China and India begin to interact as modern governments in a sustained way, bonding over their shared former status as the exploited and downtrodden of Western Imperialism and the newly-emancipated developing world. But their lack of deep ties allowed disputes to escalate, culminating in the 1962 Sino-Indian War, which left them with diplomatic differences until the early 1990s. However, that relationship has been changing rapidly. The last decade has seen a flurry of Sino-Indian diplomacy, trade and exchange, even as military tensions between the two remain substantial. The occasional border skirmish and bilateral interaction are tainted by their divergent views on relations with Pakistan, still-archrival of India and an increasingly close ally of China. This closeness between Beijing and Islamabad, coupled with a deepening skepticism in Washington over the wisdom of its own relationship with Pakistan, has pushed India and the United States closer to each other, overcoming decades of mutual suspicion as the regional dynamics change underfoot. The two are likewise engaged in an ongoing proxy struggle around the Indian Ocean and Arabian Sea, shoring up relationships with other partners. Indeed, both China and India recently signed deals with the Maldives, for investment and defense cooperation, and India has been expanding its diplomacy in Iran, traditionally an outpost of Chinese influence in the Middle East. And there is no indication that either one plans to do anything but intensify this competition in the years ahead; each has already begun to draw in other powers, from Japan to Russia to the United States. Trade has similarly intensified. Barely $2 billion fifteen years ago, it was worth a combined $80 billion last year, and continues to increase. Still, India runs a considerable and growing deficit in the relationship, much to the concern of economic officials in Delhi, who worry about the effects of cheap Chinese manufactured goods on India’s own efforts at industrialization, where China currently maintains a gigantic advantage. Indian Prime Minister Narendra Modi’s much-touted “Make in India” industrial policy has a substantial amount of ground to cover if it is to overcome the fact that the entire Indian GDP, roughly $2 trillion, is still roughly equivalent to the output of the two Chinese industrial provinces of Guangdong and Jiangsu. On military matters, India is also substantially outclassed by China at the moment. Decades of double-digit increases in military spending have made the People’s Liberation Army (PLA) budget four times as big as India’s, at nearly $215 billion to India’s $51 billion, according to SIPRI. But India has been making considerable advances of late, especially in its aircraft carrier program, where it has deeper experience than China, and importantly, in its submarine program, where it has reportedly successfully tested nuclear-capable Submarine-Launched Ballistic Missiles (SLBMs) from its nuclear-powered INS Arihant. This development would bring it a large step closer to what is often termed “second-strike capability” in nuclear deterrence, and into a kind of strategic parity with China that it currently lacks. At the same time, the geography that defined their relationship for centuries continues to benefit India in its efforts to exert influence over its eponymous Ocean, presenting a longer-term problem to China, as its economic security depends on its access to the Indian Ocean in a way that India does not depend on the Western Pacific. In all of these areas, then, diplomacy, trade and defense, China and India are bumping up against each other around the world and in their own backyards as never before. They are having to fashion a deeply multidimensional bilateral relationship almost from whole cloth in the span of years rather than decades or centuries, and doing so in the midst of a rapidly-shifting global environment. China and India are now both independent, prosperous and mostly at peace at the same time as each other, in a regional environment that is mostly secure, for the first time since the late 18th century. No one, even in Beijing or Delhi, yet knows exactly what a fully-developed relationship between China and India will look like when complete, because they’ve never seen it before. Which brings us back to the discussions currently underway to establish a military hotline between Beijing and Delhi. The fact that the Sino-Indian relationship now has enough of a foundation of cooperation that this project could be conceived is itself a measure of progress. But the fact that both countries see it as necessary underscores how much tension remains in the relationship. They cooperate with and work against each other – remaining, for the moment, the best of frenemies. Peter Marino is an analyst of Intra-Asian politics, with a particular focus on China and Indo-Pacific Security and Economic Affairs