I wrote here about the complicated relationship between the U.S. and India, during the first quarter-century after that latter country became independent in 1947.
From the late 1970s onward, the relationship improved. Three factors influenced that:
The Soviet Union's influence on India waned. Even before the USSR disintegrated in 1991, there were signs of a tilt toward the American side, late in the Cold War. When Indira Gandhi returned to power in 1980, she refused to condemn the Soviet invasion of Afghanistan. But she got along well personally with Ronald Reagan and, for the most part, the bilateral relationship was better than it had previously been. And, of course, after 1991, no country any longer had the option of playing the U.S. off against the Soviets.
India, and later Pakistan, went nuclear. India tested a nuclear device in 1974. By that time, India's rival Pakistan had begun developing its own nuclear capability. But Pakistan did not get to the stage of test explosions until 1998. The U.S. has tried economic sanctions to discourage such proliferation, but strategic considerations intervened. Seeking allies, first against the Soviet Union, and later against islamist groups, the U.S. has moderated its stance toward India and Pakistan.
India's economic growth over the past two decades has also changed the picture. For one thing, its increased dedication to capitalism puts it more ideologically in synch with America. Also, on a more concrete level, the greater the trading relationship, the more each country has at stake in its relationship with the other. Therefore, each side is more hesitant to put itself at odds with the other over any given issue.
All of these developments led to the Civil Nuclear Cooperation Agreement between India and the U.S. That agreement was reached in 2005, between Indian Prime Minister Manmohan Singh, and then-President George Bush. The U.S. Congress completed its approval process in 2008.
The agreement allows India to import materials from the U.S. for India's nuclear power plants. That is an extraordinary step for the U.S. to take with a country, such as India, that is not a party to the Nuclear Non-Proliferation Treaty. The Council on Foreign Relations provides more details here.
A horrible cliché of the current recession, "too big to fail", is applied to banks and other financial institutions. I suppose it can be adapted to the national level, by saying that, for the U.S., India has become too important to oppose.