George Orwell was highly critical of the Stalinist Regime in Soviet Russia. Explain the significance of Napoleon renaming Animal Farm, in his toast at the end of the novel, to “The Manor Farm.” Is George Orwell foretelling the eventual demise of the Soviet Union? What factors would Orwell attribute to the eventually breakdown of the Soviet Union?

In chapter ten, Napoleon changes the name of the farm from Animal Farm to The Manor Farm during a toast with Mr. Pilkington. This act is significant for a couple of reasons. First, it marks the end of the animals' vision of utopia, as imagined by Old Major during his speech in chapter one. Remember that it was Old Major who inspired the animals to overthrow Mr. Jones and take control of the farm for themselves. In addition, he envisaged a farm in which all animals were equal and united by comradeship and shared interests. When the animals overthrew Mr. Jones in Chapter two, they renamed the farm, "Animal Farm," as a sign that they had achieved this utopian dream. By changing the name back, Napoleon is making it clear that this dream is over. He is announcing to the world that he is the absolute ruler of this farm and that he will determine its name and any other matters of business.
In addition, this act is also significant because it is representative of the Tehran Conference, which took place in 1943, during the height of World War Two. In this conference, Joseph Stalin met with Winston Church and Franklin Roosevelt to discuss the best ways to defeat Hitler and the Nazis; more importantly, they met to discuss a strategy for Europe once the war had ended.
This meeting with Mr. Pilkington in chapter ten is symbolic of the congregation of these major political forces at Tehran. While the renaming of the farm is suggestive of Stalin's desire to work with Churchill and Roosevelt, there are other aspects of this meeting in which Orwell foretells the demise of the Soviet Union. The "violent quarrel" between Napoleon and Mr. Pilkington, for example, suggests that the harmony of the leaders at Tehran would not last. Notice how Orwell mentions "suspicious glances" and "furious denials," which implies that the friendship between Stalin, Churchill, and Roosevelt would be neither genuine nor long-lasting.
For Orwell, it is clear that the personal ambition of Stalin would lead to the eventual breakdown of the Soviet Union. This is shown clearly in the following line:

The source of the trouble appeared to be that Napoleon and Mr. Pilkington had each played an ace of spades simultaneously.

In other words, Napoleon's playing of the ace is symbolic of his desire to always have the upper hand and to always be one step ahead of others. Like Stalin, Napoleon would settle for nothing less than total supremacy. 
Looking deeper, Orwell's prediction was not accurate. When Stalin died in 1953, the Soviet Union did not collapse. (It did not collapse until 1991.) However, there is, perhaps, some truth in Orwell's prediction since he may have foretold the rise of nationalism. It was the desire of many of the Soviet republics, like Estonia, for example, to have a greater degree of control over their governance and internal affairs. This led to a number of nationalist movements which led directly to the fall of the Soviet Union. Had Soviet leaders, like Stalin, enabled these republics to have more autonomy from the beginning, the Soviet Union might still survive to this day.