By Paul Reynolds
World affairs correspondent, BBC News website
Britain decides to continue a nuclear weapon state
The British government's decision to replace its Trident nuclear system will have international repercussions, with arguments centred on whether this will help or hinder the spread of nuclear weapons around the world.
It comes at an extremely sensitive time for nuclear proliferation and might strengthen the self-defence arguments of countries not in the Nuclear Non-Proliferation Treaty (NPT) who have built nuclear weapons - India, Pakistan, Israel and North Korea.
Critics argue that it is incompatible with British obligations under the NPT - an argument rejected by the British government.
And they argue further that Britain is arming itself against a non-threat in that the Cold War is over and is encouraging others to adopt the general principle of keeping nuclear weapons 'just in case'.
The decision confirms that the original five countries recognised as nuclear-weapon states under the NPT - the US, Russia, China, the UK and France - have no intention of giving them up.
They argue that threats cannot be predicted and that in a world where nuclear proliferation is itself a threat, they cannot and will not take any risks.
In a White Paper accompanying the announcement, the Prime Minister Tony Blair says the "nuclear deterrent is an essential part of our insurance against the uncertainties and risk of the future."
There might also be an impact on specific policy areas, notably the effort to restrain Iran's nuclear ambitions. Iran might use the British move to argue that the West uses double standards, with Britain countering that Iran should suspend its enrichment of uranium in any event.
The argument about Trident's legality centres on Article VI of the NPT and on a ruling by the International Court of Justice in 1996.
Article VI is notoriously open to differing interpretations. This is what it says:
"Each of the Parties to the Treaty undertakes to pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament, and on a treaty on general and complete disarmament under strict and effective international control."
Does this mean, as Britain and other nuclear weapons states argue, that their obligations are met by reducing their nuclear arsenals - that is by implementing the "effective measures" towards disarmament required?
In the British case, the number of nuclear missile warheads has been reduced, this time from just under to 200 to160, and air-launched bombs given up.
Or does it mean, as critics say, that the nuclear weapon states should negotiate total nuclear disarmament and that all states should negotiate general disarmament?
International Court ruling
A ruling by the International Court of Justice in 1996 is also used by the Campaign for Nuclear Disarmament and others to argue that upgrading nuclear systems is illegal.
The court said unanimously:
"There exists an obligation to pursue in good faith and bring to a conclusion negotiations leading to nuclear disarmament in all its aspects under strict and effective international control".
However, this ruling was advisory only and its interpretation of the NPT does not have to be followed by the nuclear weapon states.
Another issue arising from the decision is the UK's reliance on the United States.
This will continue, largely because Britain will buy into the US Trident D5 missile modernisation programme. Without that the British missiles, which are serviced by the US and picked up by British submarines from a port in the US state of Georgia, will end their lives in the mid 2020s.
The US-UK nuclear relationship is enshrined in the Mutual Defence Agreement 1958 which has been renewed until 2014. This enables nuclear technology and material to be transferred both ways.
This is one fruit of the so-called 'special relationship', perhaps the main one.
A recent legal opinion argued that the agreement violated the NPT requirement to reduce nuclear armaments, but again this view is rejected by the British and US governments.
At the end of the day, the legal arguments are not proving decisive. Power politics are.