I am not at all doubting the intentions of the Soviets, just their resolve. They could never face the US in any such adventurism and they almost always backed down, especially when they faced the Americans alone. Cuban Missile Crisis is just 1 example.
Again, you and I cannot question their resolve at this juncture.
The Soviets were clear on one thing - no super power would intervene in the Indo-Pak war. If USA intervenes on behalf of Pakistan, USSR fleet would attack US fleet directly.
And it is kind of comical to even consider that the IA could mount assaults on Bangladesh that was being defended by the 7th Fleet. The idea alone would have renewed the resolve in Pakistan to continue to fight with the Mukti Bahini and IA.
There really was nothing left friend. Regardless of any amount of resolve, PA was surrounded on all sides by IA. It doesnt matter what their resolve was.
USSR clearly stated that any physical USN intervention in the scenario would be retaliated with a strike on USN fleet.
India had just signed a Treaty with USSR a few months before. It stated that any attack on either of the two would be construed as an attack on both of them.
The US government decided that they were not bluffing and took a rational call that they would not risk their own men and women for Pakistan. You also have to understand that the US Government was under pressure to stop Pakistan's repression in East Pakistan. The Blood telegram sent by US Ambassador in Pakistan to White House also put pressure on them.
They factored all the issues and decided that its best to not intervene. What you are asking now is that US should have fought Pakistan's war, sacrificed its men for Pakistan in a situation when Pakistan was carrying out repression on the local population.
They must have had their reasons, whatever be the case. Even China would have engaged the Indians had the US initiated such a process. Would the Soviets have taken on both China and US for India???
Think about it.
As a matter of fact US did ask China to start sabre rattling.
The thing is , India factored China before the war began.
Indira Gandhi wanted to invade East Pakistan as early as in Summers/Rainy Season.
It was General Manekshaw who changed the plans. He postponed the invasion to Winters for 2 reasons. He explained both clearly to Indira Gandhi. All these facts are documented.
1. The winters would mean that all the passes in the North would get completely blocked off. That means China would not be able to intervene in any meaningful way. The geography of Indo-China border is that Himalayas can only be crossed when the passes are open.
2. During monsoons, the Rivers in BD become massive marshes(BD is like a delta), that would quadruple the difficulties Indian Army would face while invading. This would not be the case in Winters.
These were the two main reasons IA invaded as late as it did.