Actually, US tried every possible trick under the sun to prevent India from developing nuclear weapons. But India managed to find a way around every roadblock placed by US in its way.
The nuclear program of India started under the leadership of PM Nehru and Dr. Homi Bhabha (Indian "Oppenheimer") in the late 1950s. Documents obtained by Jeffrey T. Richelson from the US National Security Archives show that as early as 1958, the CIA was concerned about the possibility that India might choose to develop nuclear weapons.
In the early 1960s, the CIA told Indian political leaders (Nehru and Shastri) as well as Dr. Bhabha in no uncertain terms to stop their pursuit of the nuclear technology. Bhabha told the CIA guys to mind their own business. But CIA's business at that time was to stop India's nuclear program!
On Jan. 24, 1966, when Dr. Bhabha was traveling in an Air India plane over Mont Blanc (France), the CIA arranged a bomb to go off in its cargo hold, killing Bhabha and 116 others. This claim is based on an interview given by the then CIA "director of plans" Robert Crowley to journalist Gregory Douglas (see link below).
Although Crowley's claim in this interview is clearly debatable, I tend to believe him. There are many reasons for my belief, one of which is that assassination of unfriendly foreign leaders and foreign nuclear scientists was a common practice for CIA in 1960s. For example, CIA has officially acknowledged having made several attempts on Fidel Castro's life in the 60s. And even in the 21st century, many Iranian nuclear scientists have been assassinated by CIA and/or Mossad.
Although Bhabha's death did set back India's nuclear program somewhat, India managed to conduct its first "crude" nuclear test (code named Smiling Buddha) in 1974. This test completely surprised the US. Though Indira Gandhi claimed that the test was for peaceful purposes, the US and its western allies rushed to impose all sorts of nuclear and other technological sanctions on India. By the late 1980s, however, India was ready to conduct some more sophisticated nuclear tests. However, India was very concerned about the international reaction. In particular, India's foreign exchange reserves were in bad shape, and by 1991 India had just enough reserves for one month's worth of imports.
In the 1990s, India kept looking for the right political window to conduct the test. The PM who ordered the tests had to be bold. The logistic preparations for the test had to be done secretly to prevent CIA from throwing a roadblock. And India had to be prepared to face the international economic sanctions that were sure to follow.
NPT and CTBT
The NPT was the basic strategy of US and other nuclear powers to prevent the horizontal proliferation of nuclear weapons. However, it did nothing to prevent the vertical proliferation of nuclear weapons by the five nuclear powers themselves. Many poor developing countries were bribed and bullied into signing the NPT. But India firmly rejected the NPT because it was blatantly discriminatory towards non-nuclear states.
Another major tactic of US to prevent India from acquiring nuclear weapons was the CTBT (Comprehensive Nuclear-Test-Ban Treaty). Basically, by early 1990s, the five super powers had done all the nuclear testing they needed to do, so they tried to impose a worldwide ban on nuclear testing. This would have effectively shut the door for new entrants to the nuclear club. But India fought valiantly to prevent the passage of CTBT. In 1996, India's representative in Geneva declared that India would "never sign this unequal treaty, not now, not ever." This was clear evidence that India had every intention of conducting a nuclear test in the foreseeable future. The CTBT treaty remains unratified by many key countries as of today.
In May 1998, prime minister Vajpayee finally ordered the nuclear tests that placed India into the exclusive nuclear club.
