The UGM-133A Trident II, or Trident D5 is a submarine-launched ballistic missile (SLBM), built by Lockheed Martin Space in Sunnyvale, California, and deployed with the United States Navy and Royal Navy. It was first deployed in March 1990, and remains in service. The Trident II Strategic Weapons System is an improved SLBM with greater accuracy, payload, and range than the earlier Trident C-4. It is a key element of the U.S. strategic nuclear triad and strengthens U.S. strategic deterrence. The Trident II is considered to be a durable sea-based system capable of engaging many targets. It has payload flexibility that can accommodate various treaty requirements, such as New START. The Trident II's increased payload allows nuclear deterrence to be accomplished with fewer submarines, and its high accuracy—approaching that of land-based missiles—enables it to be used as a first strike weapon.
Trident II missiles are carried by 14 US Ohio and 4 British Vanguard-class submarines, with 20 missiles on each Ohio class and 16 missiles on each Vanguard class (the number of missiles on Ohio-class submarines was reduced from 24 to 20 by 2017, in compliance with the New Strategic Arms Reduction Treaty). There have been 215 total test launches of the D5, with 207 successes. 196 launches were from the sea: 191 successes and 5 failures. 181 of the successes and 3 of the failures were by the US, while 10 of the successes and 2 of the failures were by the UK. 19 launches were from land, all by the US, with 16 successes and 3 failures. the most recent successful launch from USS Louisiana (SSBN-743) on 27 September 2023. There have been 8 test flights that were failures, the most recent being from HMS Vanguard off the coast of Florida in January 2024. The D5 is the sixth in a series of missile generations deployed since the sea-based deterrent program began 60 years ago. The Trident D5LE (life-extension) version will remain in service until 2042.