Defence Secretary Ben Wallace has said he does not expect to become the next Nato chief.
Speaking to the Economist, the Conservative MP said "it's not going to happen" and there are "a lot of unresolved issues in Nato". He added that he thinks the United States wants the current secretary general, Jens Stoltenberg, to stay in post for another year.
Stoltenberg, a former prime minister of Norway, is due to step down in September after nine years as head of the military alliance, but its members are yet to decide on a replacement. Wallace had been rumoured as a contender for the job in recognition of the support he gave Ukraine after Russia's invasion.
Earlier in the month, US President Joe Biden said he would support a Nato leader from the UK when he met Rishi Sunak at the White House.
Speaking to a German news agency about the job last month, Mr Wallace said: "I've always said it would be a good job. That's a job I'd like. But I'm also loving the job I do now."
So what is Nato and which countries are already members?
Nato - the North Atlantic Treaty Organisation - is a political and military alliance formed of 30 mostly European countries, as well as the US and Canada. Russia and Ukraine are not members.
The heart of the Nato alliance is Article 5, an agreement that an armed attack on one member will be viewed as an attack on all, and that they are obliged to defend one another.
Nato was formed by 12 countries in 1949 to counter the threat of post-war communist Russian expansion in Europe. In 1955, Soviet Russia responded to Nato's formation by creating its own military alliance of eastern European communist countries, called the Warsaw Pact.
After the collapse of the Soviet Union in 1991, several former Warsaw Pact countries switched sides and became Nato members.
Nato does not have its own armed forces. Instead, it has a military command structure that works with the militaries of member countries in peacekeeping operations.
Nato is made up of 28 European nations, as well as the US and Canada.
The founding members were:
Members who have joined since are:
Nato said it condemned "in the strongest possible terms" Russia's attack on Ukraine, but it has not sent any troops there.
Ukraine is not a member of Nato, so the alliance was not obliged to defend it. But, the Nato secretary general has now confirmed that Ukraine will become a member of the organisation once the war ends.
He said in a news conference: "Let me be clear, Ukraine's rightful place is in the Euro-Atlantic family. Ukraine's rightful place is in Nato."
Since the Russian invasion of Ukraine, member countries have been supporting Ukraine by sending tanks and defence equipment.
Russia vehemently opposes Ukraine's potential membership of Nato. It has demanded a formal veto on it ever becoming a member - something the alliance has refused to agree to.
Russia fears Nato has been encroaching on it by taking on new members in eastern Europe. It said that admitting Ukraine would bring Nato forces into its backyard. Nato denies that this is a valid concern since it is a peacekeeping alliance.
In December 2021, Mr Putin said Russia would seek "reliable and long-term security guarantees" from the US and its allies "that would exclude any further Nato moves eastward and the deployment of weapons systems that threaten us in close vicinity to Russian territory".
More broadly, Mr Putin wants Nato to pull back its existing military presence in eastern Europe. This includes a regularly rotating series of exercises in Lithuania, Latvia and Estonia, all former Soviet states.
Six Nato countries currently border Russia or the Russian exclave of Kaliningrad: Estonia, Finland, Latvia, Lithuania, Norway and Poland.
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