The US supported India in the Sino-Indian war and such, but not long after they threatened India with nuclear weapons during the 1971 war. What happened to cause the relationship to sour? And how is the relationship now?
My understanding is that the United States never had a very good diplomatic relationship with the post-1947 Republic of India.
Thoughout the cold war, the fact that India kept trying to remain diplomatically non-Aligned while simultaneously purchasing Soviet-bloc military hardware made the United States foreign service very suspicious of Indian motives and allegiances.
After India started buying Soviet military hardware, the United States felt compelled to support Pakistan in supply training and weapons. This was part of the international politics of the Cold War -- any conflict that the Soviets were supporting one side, the Americans felt compelled to support the other side, no matter how distasteful they were in other ways.
In the 1970s, Richard Nixon was trying to split China away from the Soviet Union, which meant that too friendly a U.S. stance toward India might hurt overtures toward China. In contrast, ten years earlier, China had been very strongly allied with the Soviet Union, so the United States had nothing to lose and everything to gain by supporting India in the Sino-Indian war.
B/c India is getting strong now & they want a regional power to control China. Pakistan is gotten weaker b/c of terrorism but things are getting better.