In response to Worst Case War Scenario the comment was made that a portion of the scenario was off because India would not fight against Russia.
It is true that, for a while during the Cold War, India and Russia were close to each other. This was a balance to closeness between Pakistan and the United States, which was part of an encircling attempt around Iran. This does not mean that India wouldn’t fight on the side of the United States in a worst case war scenario.
A key point in the scenario is that China and Russia are allies against the United States. However, there was a time during the Cold War, when China and the Soviet Union were very close to going to war against each other. That was part of the strategy behind President Nixon visiting China, as part of the plan to eventually pull out of Vietnam. By visiting China, President Nixon caused concern in the Soviet government that the United States and Chine might coordinate against the Soviet Union.
But as bitter enemies the United States and the Soviet Union were during the Cold War, they were allies during World War Two. The Cold War blossomed under President Truman as a way to justify intervening in Greece, turning a former ally into an enemy.
In World War Two, Germany and Japan were the main enemies of the United States. Today both are considered friendly countries. Germany in particular is now the anchor to the European Union with many former enemies.
Political alliances shift constantly. It is part of the nature of the game. Just because two governments were enemies doesn’t mean that they cannot shift to allies, and just because two governments were allies doesn’t mean that they cannot shift to enemies. George Orwell noticed this, which is why he included it in "1984" when he had Oceana constantly shift between alliance with Euraisa and alliance with Eastasia.
So, why would India side with the United States against Russia?
There exists tension between India on the one hand, and both Pakistan and China on the other. There has been concern at times that if nuclear war were to break out it would be in the Himalayan Mountain range, either between India and Pakistan or India and China. There is still disputed territory at the intersection of those countries, and religious conflict is particularly strong between the people of India and Pakistan. Plus, relations are souring between Pakistan and the United States while relations are improving between Pakistan and China.
If India were to intervene on the side of the United States, going against Russia would be incidental instead of purposeful. Just as Finland, by being an enemy of the Soviet Union, was an enemy of chance against Great Britain and the other allied powers, India would be fighting primarily against Pakistan and China and not primarily against Russia.
Pakistan and China would both be against the United States in that scenario. That would mean, even if India doesn’t intervene, that India would be closer to the side of the United States and therefore farther from the side of Russia. If India doesn’t intervene, that would make that country a silent ally instead of active ally of the United States, similar to the role played by Spain to Germany in World War Two.
Of all the parts of the scenario, that is the one with the lowest probability. Just because India would be favoring the United States and experiencing poor relations with both Pakistan and China doesn’t mean that intervention has to happen. The government of India has the least cause of all the countries mentioned in the scenario. That could mean that some sense would break out, the government of India could choose to sit out the war, and therefore emerge as very dominant in the area after China, Russia, and the United States finish tearing each other apart. But if it were to intervene, it would not be on the same side as China and Pakistan, and therefore on the opposite side of Russia.