UGM-133A三叉戟II型弹道导弹，又称三叉戟D5是一款潜射弹道导弹（SLBM），由洛克希德·马丁公司位于加州森尼韦尔的空间系统公司研发制造，装备美国以及英国海军。三叉戟II型于1990年首次服役[4]，是三叉戟I型的改进型号，打击精度更高，载荷、航程也均有增加。是美国核三位一体重要的一环，也是美国核威慑重要的组成部分。三叉戟II型是具有威慑力的海基弹道导弹，也是美国在新削减战略武器条约的谈判过程中有力的筹码。由于弹头载荷的增大，美国军方只需要更少的核潜艇就能达成同样的核威慑力[10]，且由于及精度很高，接近陆基弹道导弹，三叉戟II型可用于一次打击。[11][12][13]
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目前搭载三叉戟II型的潜艇有14艘美国海军的俄亥俄级核潜艇以及4艘英国海军前卫级核潜艇，每艘俄亥俄级能搭载24枚，前卫级能搭载16枚（因美俄签署了新削减战略武器条约，俄亥俄级把搭载数量减少到20枚）。1989年设计完成后，已经有161次成功的试射。[14]最近一次试射由俄亥俄级内布拉斯加号于2018年3月执行。[15]另外大约有少于10次的发射失败[16]，上一次发射失败是2016年6月份，英国皇家海军前卫级复仇号潜艇_(S31)发射三叉戟II型失败[17]。2002年美国海军公布延寿计划， 三叉戟II型预计将服役至2040年。[18]
Parsch, Andreas. Trident D-5. Encyclopedia Astronautica. [11 June 2014]. （原始内容存档于2018-09-24）. Parsch, Andreas. UGM-133. Directory of U.S. Military Rockets and Missiles. [11 June 2014]. （原始内容存档于2019-05-14）. Lieber, Keir A.; Press, Daryl G. U.S. Nuclear Primacy and the Future of the Chinese Deterrent (PDF). China Security: 77. 2007 [2018-09-25]. （原始内容存档 (PDF)于2017-05-16）. During the past 15 years, the United States has done so much to upgrade its first strike capabilities – most notably by deploying Trident II D-5 missiles throughout the entire ballistic missile submarine (SSBN) fleet, placing high-yield W88 warheads on many of those missiles, and deploying stealthy B-2 bombers – that today a first strike could succeed even if the performance of key U.S. weapon systems fell far short of their expected accuracy, reliability, or both.
Cimbala, Stephen J. Military persuasion: Deterrence and provocation in crisis and war. Penn State Press: 85–6. 2010 [29 January 2016]. （原始内容存档于2018-09-26）. By the end of the 1980s, however, the submarine-launched ballistic missile had turned another page. The accuracy of the Trident II (D-5) SLBM, planned as the replacement for the Trident I with Trident II deployments beginning in 1989, was comparable to that of the MX/Peacekeeper ICBM, the most accurate land-based missile in the U.S. strategic arsenal. Owing to its improved accuracy and larger payload compared to its SLBM predecessors, Trident II would be able to attack hardened targets in the Soviet Union that were not previously vulnerable to sea-launched ballistic missiles. Although U.S. planners might assume that these strikes against hardened targets in the Soviet Union would be retaliatory attacks, a Soviet net-assessment of U.S. first-strike capabilities would have to include the improved sea-based missiles.
Stellan Vinthagen. Tackling Trident. Irene Publishing: 41. 2012 [29 November 2017]. （原始内容存档于2018-09-26）. Although it is acurate enough for a 'first strike' weapon, successive governments have been adamant that the purpose of the current Trident system is as a 'deterrent' against nuclear or similarly cataclysmic attack on Britain. The Trident 'mission' is outlined by the Ministry of Defense: 'In a posture known as Continuous At Sea Defence (CASD), one submarine, armed with up to 16 Trident missiles and up to 48 warheads, is always on deterrent patrol 24 hours a day, 365 days a year' (MoD, 2006).
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