India's nearly completed, $5.43 billion purchase of Russian S-400 air-defence systems raises serious obstacles to closer politico-military relations between Washington and New Delhi. It requires rigorous strategic thinking to avoid hampering deeper policy relationships within the Asian "Quad" (the U.S., India, Japan and Australia), compromising America's stealth technology or jeopardizing seemingly mundane but often critical issues of interoperability among national militaries. Finding mutually acceptable solutions has enormous implications; so does failure.

Undoubtedly, India needs advanced air defences. It has long, difficult-to-defend borders with China; Beijing's growing navy is increasingly menacing, as are Pakistan's nuclear and ballistic-missile programs, fostered by China.

But India's S-400 purchase, formalized in October 2018, was a mistake, even from its own strategic perspective. New Delhi directly challenged earlier U.S. legislation intended to block significant Russian weapons sales, and which provided very limited presidential waiver authority. Especially unfathomable in why India would acquire the same system China was buying, risking that Beijing's cyber warriors, perhaps exploiting Moscow-inserted back doors, could cripple their defences in a crisis. Turkey's similar purchase of S-400s, and the dynamics among the three transactions, bear particularly on the current campaign to waive sanctions against India.

Washington sanctioned Beijing in September 2018 with broad U.S. domestic support. Turkey's acquisition provoked considerable controversy, coming as it did from a NATO ally. S-400s are, not surprisingly, completely incompatible with NATO-wide air defence capabilities, leaving the alliance's south-eastern flank potentially vulnerable. (A humorous contemporaneous remark was that Turkish President Recep Erdogan wanted the S-400s to defend himself against Ankara's own air force.)

In addition, Turkey co-produced components of the stealthy F-35 and had ordered 100 of them. Significant exposure of F-35s to S-400 radars would give the air-defence operator a clear advantage in detecting F-35s despite their stealth, thereby possibly fatally compromising the entire F-35 program. After extended debate, President Trump reluctantly and belatedly ejected Turkey from the F-35 program in 2020 and imposed economic sanctions. To this day, the potential proximity of U.S. F-35s and Russian S-400s in Turkey arouses concern.

Perhaps bolstered by Trump's evident reluctance to punish Turkey and equally evident divisions among Trump's advisers, India's decision to proceed nonetheless reflects a backward-looking dependence on Russia for sophisticated aerospace and weapons technology. Now, with deliveries imminent, Indian sources still argue that the deal shouldn't be cancelled: The actual agreement was in 2016 (before the sanctions legislation), India is dependent on Russia for spare parts and maintenance under previous weapons-systems contracts and imposing sanctions would push New Delhi back toward Moscow.

These are arguments of inertia and complacency, and they should carry no weight for the U.S. Vague assertions about future conduct, even accompanied by reduced reliance on major purchases from Russia, are insufficient to risk undermining our global efforts to counter the spread of Kremlin arms sales. Having New Delhi and Washington grow closer means just that, not equivocating or reversing field.

In fact, India's direction in foreign arms purchases is decidedly unclear. Last week, its ambassador to Russia, Bala Venkatesh Varma, said that "there has been a fundamental change in how our defence relationship has moved in the last three years. Russia has moved back again as the top defence partner of India." Still worse are reports that, even before the initial S-400 purchases are fully deployed, India and China are considering upgrading to the new S-500 system.

Skeptics might say New Delhi is playing Washington. Even viewed benignly, India is sending contradictory signals, likely due to competing views inside its government and body politic. Whatever Prime Minister Narendra Modi's reasons, the other Quad members have compelling reasons for New Delhi to articulate its future defence-procurement strategies more precisely. No one need commit to a full-blown, politico-military alliance to see the importance of striving for interoperability among like-minded states before things go further, if they ever do. NATO struggled with interoperability problems for decades, thereby leaving the alliance less effective, operationally and as a deterrent. There is no reason to engender potential problems, which prudent planning could avoid.

In such circumstances, any U.S. waiver for India's S-400 purchases must come with clear conditions and requirements. Pending legislation in Congress says merely that the president may not impose sanctions upon a Quad member unless he "certifies ... that that government is not participating in quadrilateral cooperation ... on security matters that are critical to the United States' strategic interests." That is no condition at all; if those were the facts, it would mean there was no Quad, but merely a Trio.

Developing U.S. conditions for the waiver is an urgent priority. Washington should at least require an agreed-upon timeline and metrics to reduce Indian purchases of sophisticated Russian weapons systems, regular Quad consultations on meeting these targets and more extensive politico-military planning for Indo-Pacific threats, thereby shaping future procurement requirements.

We need not insist that India acquire all its future high-end weapons systems from the U.S., although it would obviously be helpful to see larger purchases than at present. Many Western countries are capable of supplying Indian needs, further highlighting the advantages of breaking the Russian mould. America, Japan, Australia and others also could offer opportunities for defence cooperation with India along the lines of the AUKUS project on nuclear-powered submarines, to enhance India's own domestic weapons productions.

This model is important not only for the Indo-U.S. relationship but for many others, including Turkey. If sanctions waivers or general lassitude regarding Russian weapons sales and their consequences for regional balances of power become commonplace in Washington, the problem will continue to grow. It is entirely certain that an Indian waiver will trigger instant demands for like treatment from Turkey and other prospective purchasers, while enabling Rosoboronexport, Russia's foreign-military-sales agency, to exploit our lack of willpower. Ironically, Turkey might warrant a waiver, with appropriate conditions, if the Turks remove Erdogan from office in upcoming elections, so resolving the India problem could well be precedential.

Decisions of this magnitude require Washington to pursue a conscious strategic approach, rather than simply treating an Indian waiver (or any other) as a one-off. Time is short.