"Recently the WikiLeaks website leaked that the alliance was formed due to the pressure from the Americans," Bose said at a press conference here. 

He recalled that 61 Left front MPs, including 35 from West Bengal, had opposed UPA-1's nuclear policy, "in which US interests were involved".

The Left had withdrawn its support to the UPA-1 government over the India-US civil nuclear deal, prompting it to seek a confidence vote in parliament in July 2008.According to Bose, the US "decided to take revenge against the Left Front in Bengal and the alliance between the Congress and the Trinamool Congress was formed".