the fourth-largest defence budget in the world. For comparison's sake, this sees Britain spending more in absolute terms than France, Germany, India or Japan, a similar amount to that of Russia, but less than China, Saudi Arabia or the United States. In September 2011, according to Professor Malcolm Chalmers of the Royal United Services Institute, current "planned levels of defence spending should be enough for the United Kingdom to maintain its position as one of the world's top military powers, as well as being one of NATO-Europe's top military powers. Its edge – not least its qualitative edge – in relation to rising Asian powers seems set to erode, but will remain significant well into the 2020s, and possibly beyond." The Strategic Defence and Security Review 2015 committed to spending 2% of GDP on defence and announced a £178 billion investment over ten years in new equipment and capabilities. Nuclear weapons The United Kingdom is one of five recognised nuclear weapon states under the Non-Proliferation Treaty and maintains an independent nuclear deterrent, currently consisting of four ballistic missile submarines, UGM-133 Trident II submarine-launched ballistic missiles, and 160 operational thermonuclear warheads. This is known as Trident in both public and political discourse (with nomenclature taken after the UGM-133 Trident II ballistic missile). Trident is operated by the Royal Navy Submarine Service, charged with delivering a 'Continuous At-Sea Deterrent' (CASD) capability, whereby one of the Vanguard-class strategic submarines is always on patrol. According to the British Government, since the introduction of Polaris (Tridents predecessor) in the 1960s, from April 1969 "the Royal Navy's ballistic missile boats have not missed a single day on patrol", giving what the Defence Council described in 1980 as a deterrent "effectively invulnerable to pre-emptive attack". As of 2015, it has been British Government policy for the Vanguard-class strategic submarines to carry no more than 40 nuclear warheads, delivered by eight UGM-133 Trident II ballistic missiles. In contrast with the other recognised nuclear weapon states, the United Kingdom operates only a submarine-based delivery system, having decommissioned its tactical WE.177 free-fall bombs in 1998. The House of Commons voted on 18 July 2016 in favour of replacing the Vanguard-class submarines with a new generation of s. The programme will also contribute to extending the life of the UGM-133 Trident II ballistic missiles and modernise the infrastructure associated with the CASD. Former weapons of mass destruction possessed by the United