Geopolitics is the best determinant of whether relations between India and the United States are on an even keel. Not personalities, not economic relations and not the number of lobbyists hired in Washington.
Consider the Indo-US relationship during the Cold War.
The first six years of the Eisenhower years were marked by indifference if not hostility to New Delhi. The last two years both sides were best friends: US aid to India surged, Eisenhower had a wildly successful visit to New Delhi.
This bonhomie continued under the 1000 days of Kennedy rule. But under Lyndon B. Johnson, an administration that was staffed entirely by Kennedy men, the two sides begin to drift apart again.
In 1965 this seven-year plateau in Indo-US relations came to an end with New Delhi signing a five-year defence agreement with the Soviet Union.
The Indian prime minister during this entire period was the same man: Jawaharlal Nehru. And his policies remained largely unchanged. What made Eisenhower belatedly love India, Kennedy stay enamoured and Johnson lose interest? The long answer: geopolitics. The short one: China.
India used to berate the US over its anti-communism. But after China overran Tibet in 1958, Nehru and Eisenhower suddenly found reasons to be closer. The handshake became an embrace when the 1962 war broke out. India and the US were on the same side when it came to Beijing, though continuing to differ on Moscow.
Johnson became entangled in the Vietnam war. He argued this was a proxy war against Maoist China. Nehru saw things differently. When Johnson refused to supply the arms that Kennedy had promised, after 1962 India turned to Moscow for the weapons it needed against China. The point is that the perceived threat of Maoist China explained why these various US presidents changed their attitudes towards India and vice versa. Personalities mattered little. Geopolitics was everything. This remains the prime determinant in Indo-US relations.
The late 1960s and 1970s were the bleakest period in bilateral ties because there was no geopolitical match. That Richard Nixon and Indira Gandhi didn't like each other didn't help. But they would have swallowed their bile if their worldviews matched.
The US, seeing an opportunity in the Sino-Soviet split, sought a quasi-alliance with China against the Soviet Union. India still saw China as threat number one and the Soviet Union as friend number one. The twain could not only meet, it was heading in opposite directions - as became evident in the 1971 Indo-Pakistan war.
The Jimmy Carter and Ronald Reagan years were not marked by much Indo-US hostility, but their wasn't much friendliness either. New Delhi and Washington simply didn't have much to talk about. There wasn't any geopolitical friction, but there was no overlap either. End result: no US president visited India for a remarkable 22 years during this time.
Now US presidents are flying in quite regularly.
However, with the Indian economy and the desi diaspora, can it be said the primacy of great power politics is over? Bill Clinton, after all, didn't have a big strategic game plan when he came to India. George W. Bush, who took "the balance of power for freedom" seriously, did: he wanted democratic India to rise better and faster than one-party China. Obama has come twice though his geopolitical vision is about getting America out of the geopolitical business altogether.
Closer scrutiny indicates it's still a major issue. Clinton was friendly but never actually did anything for India. Bush, geopolitically taken with India, moved mountains on behalf of New Delhi. Strategically confused Obama has gone back and forth on India. Which is why international relations experts look first at the big picture and only then at a Devyani Khobragade or a Pakistani military dole. If the big picture is geopolitically sound, it trumps everything else. And the Indo-US big picture remains a charcoal sketch, awaiting more layers of paint.