Given the task definition and input, reply with output. In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: The War Game depicts the prelude to, and the immediate weeks of the aftermath of, a Soviet nuclear attack against Britain. The narrator says that Britain's current nuclear deterrent policy threatens a would-be aggressor with devastation from Vulcan and Victor nuclear bombers of the British V bomber force.
The film begins on Friday, 16 September (presumably 1966; this date wouldn't appear again until 1977). A news report tells of a Chinese invasion of South Vietnam; tensions escalate when the United States authorises tactical nuclear warfare against the Chinese. Although Soviet and East German forces threaten to invade West Berlin if the US does not withdraw, the US does not acquiesce to communist demands and the invasion takes place; two US Army divisions attempt to fight their way into Berlin to counter this, but the Russian and East German forces overwhelm them in conventional battle. In order to turn the tide, President Johnson authorises the NATO commanders to use their tactical nuclear weapons, and they soon do so. An escalating nuclear war results, during which larger Soviet strategic IRBMs are launched at Britain. The film remarks that many Soviet missiles were, at the time, believed to be liquid-fuelled and stored above ground, making them vulnerable to attack and bombings. It hypothesises that in any nuclear crisis, the Soviet Union would be obliged to fire all of them as early as possible in order to avoid their destruction by counterattack, hence the rapid progression from tactical to strategic nuclear exchange.
Who are the communists demanding for the US to withdraw?