Why does China need to build a huge nuclear force for a counter-strike?
1. There is an expected failure rate. Though China has a very good record in the reliability of their civilian rockets/missile analogs (see
China Matches U.S. Space Launches for First Time | Danger Room | Wired.com), some of the DF-5 ICBMs were built a long time ago.
2. Part of China's counter-strike missiles is expected to be shot down by U.S. conventional anti-ballistic missile defense.
3. In a surprise U.S. attack (e.g. U.S. has never agreed to a pledge of No First Use), a first strike is expected to destroy a significant portion of China's counter-strike missiles.
4. Some of China's counter-strike missiles will be neutralized by U.S. nuclear-tipped anti-missile missiles.
U.S. Nike Hercules nuclear-tipped anti-missile missile with a 2 to 20 kiloton warhead and a range of 77 miles (or 124 km)
High-altitude nuclear weapon test:
5. MIRVed warheads on the DF-41 ICBMs and Julang-2 SLBMs are expected to be only 150 kilotons each. Therefore, using the standard from the U.S. SIOP plan (see
Why Obama Can't End Nukes - Newsweek), five warheads are targeted at each city to ensure its destruction.
6. This is politically incorrect, but I'm going to say it anyway. An all-out nuke war isn't just between two countries. Once a global thermonuclear war is underway, every major and medium-sized city in the world will be targeted. After all, you can't let some minor country become a superpower in a post-nuclear-war world by leaving its cities intact. To accomplish this strategic objective, a massive nuclear arsenal is necessary.
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Regarding the issue of the number of Chinese silos, I can assure you that the claim of only 20 Chinese ICBM silos is ludicrous. The length of a DF-21 IRBM is 11m. The length of a DF-31 ICBM is 13m. If a silo is dug a little deeper and wider, it can accommodate an ICBM; instead of an IRBM.
In the following video, which encompasses only a few mountains, I counted at least 30 silos. We know from a Chinese-state television CCTV broadcast on March 24, 2008 that China has built a 5,000km (or 3,000-mile) missile complex under a mountain range. If a few mountains contain 30 silos, imagine how many silos are hidden along 5,000km.