Admiral Sir Tony Radakin has taken over from General Sir Nick Carter as the Chief of the Defence Staff.
General Carter had been expected to retire in June but was asked by Prime Minister Boris Johnson to stay in post until late 2021.
Admiral Sir Tony Radakin has spent 29 months at the helm of the Royal Navy, but what are some of the main issues he faces as he begins his new role as UK military chief?
Russia and Eastern Europe are becoming an increasing area of concern for British and NATO troops.
NATO says it is "closely monitoring" as Russia builds its military presence near its border with Ukraine.
The Ukrainian president has alleged his country's intelligence service has uncovered plans for a Russia-backed coup d'etat against him set for next week.
In recent weeks, Ukrainian and Western officials have expressed concern that a Russian military build-up near Ukraine could signal plans by Moscow to invade its ex-Soviet neighbour.
The Kremlin insists it has no such intention and has accused Ukraine and its Western backers of making the claims to cover up their own allegedly aggressive designs.
NATO secretary-general Jens Stoltenberg warned Russia on Friday that any attempt to invade Ukraine will have consequences.
Amid concerns over rising tensions with Russia, the UK entered a new deal with Ukraine, which will see British warships and missiles sold to Kiev.
British troops have been deployed to Poland's border with Belarus to address the migrant crisis there.
Watch: NATO and EU agree to step up work in countering threats posed by Belarus and Russia.
Western governments have accused Belarusian President Alexander Lukashenko, a close ally of Russian President Vladimir Putin, of deliberately encouraging the migrants to breach its borders in retaliation for EU sanctions.
The UK's Carrier Strike Group, led by Royal Navy aircraft carrier HMS Queen Elizabeth, had more than 30 live interactions with the Russian military during its recent deployment.
Recent NATO-Russia tensions prompted Britain to preposition armour and equipment in continental Europe, with the creation of a new Defence Global Hub in Germany.
The UK and the West watch closely as relations between China and Taiwan appear to be growing more tense.
Taiwan has said 27 Chinese aircraft entered its air defence buffer zone on Sunday, the latest in a long series of incursions as part of Beijing's pressure on the self-ruled island.
Over the past year, the frequency of Chinese incursions has increased, with about 150 aircraft crossing into Taiwan's buffer zone over a period of four days.
China has also been accusing the United States of "flexing muscle and stirring up trouble in the Taiwan Strait repeatedly in the name of freedom of navigation".
Beijing protested about the passage of US Navy destroyer USS Milius through the Taiwan Strait.
In the summer, China also sent an icy warning to the UK Carrier Strike Group (CSG21) upon its arrival in the contentious South China Sea, saying it is ready to deal with "improper acts".
With the Western drawdown of troops from Afghanistan, how might the Taliban seek to exert their influence on the region's stability and could the state become a harbour for international terror actors?
Meanwhile, Gulf nation Oman is to become a strategically important training hub for the British military.
It is set to be designated one of the Army's Land Regional Hubs – which will support the training of expeditionary forces.
Tensions between Iran and the West continue to boil.
Just ahead of nuclear talks with the West, Iran's military began a show of force with annual war games, according to its state TV.
Reports said navy, ground and air force units were participating in the exercise in a coastal zone in the Gulf of Oman, in early November.
Watch: Radicalisation in Africa likely to be 'inspired' by Afghanistan.
Could Africa too become a flashpoint of terror?
Adm Radakin will have to consider the UK's role there, especially in the infamously dangerous Sahel region.
British troops are deployed to the region in Mali, serving as part of a UN peacekeeping mission – described as the most dangerous peacekeeping mission in the world.
During the deployment in October, UK soldiers came under fire for the first time since 2014 and responded, killing two fighters from a 'terrorist armed group', the Ministry of Defence (MOD) said.
In November, personnel also detained three suspected so-called Islamic State fighters and seized weapons.
Defence Secretary Ben Wallace spoke to the Lord's International Relations and Defence Committee in October about the importance of security in Africa.
"You don't have to go to the Pacific to compete with China, you only have to go to Africa," he said.
"You don't have to go to Russia to compete with Russia, you only have to go to Africa.
"So a lot of these global challenges are actually being played out in Africa.
"The spread of radicalisation is very quick at the moment."
Watch: Who is Admiral Sir Tony Radakin?
Futureproofing the Armed Forces
The constant demands for cutting-edge strategy, equipment and facilities mean Admiral Radakin will have a close eye on areas to improve the military's efficacy on the battlefield and in public.
The British Army's new elite Ranger Regiment, under the umbrella of a new Special Operations Brigade, seeks to make the UK's land forces more efficient but also more lethal.
The regiment is part of 'Future Soldier' – the most radical transformation programme for the service in more than 20 years.
It is said to demonstrate how the Army is modernising to address future threats from across the globe – following on from the Integrated Review earlier this year.
Meanwhile, Royal Marines are pioneering 'game-changer' technology to help eliminate the risk of potentially fatal heat illness as recruits push themselves to the limit.
There then is the question of how to utilise older facilities or training areas – should they be revamped, or consigned to the past?
Mr Wallace has said the British Army's training base in Canada will see "change" but will not be closing, as Britain looks to develop five global 'land hubs'.
Watch: Royal Marines pioneer 'game-changer' technology to eliminate heat illness.
Our footprint on the natural world continues to influence military planning and future strategy.
The RAF is "ready" to use green aviation fuels more if they are "readily available", the Chief of the Air Staff has said.
When asked why the RAF isn't using eco-friendly fuels as much as other air forces, Air Chief Marshal Sir Mike Wigston told Forces News "we're going as fast as the global sustainable fuel production industry is making it".
Just a week before his comments, the RAF's VIP Voyager aircraft achieved a "historic first" as it flew the Prince of Wales and Duchess of Cornwall to Jordan using sustainable aviation fuel which is said to be able to cut carbon emissions by up to 80%.
A Royal Air Force test pilot also recently made history with the world's first-ever flight on 100% synthetic fuel.
Defence, including the RAF, is making a number of changes to improve its sustainability as the UK works to be carbon net-zero by 2050.
Watch: RAF chief – 'The time is now to get on with' achieving net-zero carbon emissions.
Defence chiefs and politicians have been frank when addressing the sobering reports and revelations of mistreatment of personnel linked to 'laddish' culture.
A British Army soldier was recently jailed and kicked out of the military after he forced junior recruits to box each other in the nude during an initiation ceremony.
Rifleman Kieran Trewin, 26, admitted running the humiliating event, described as naked "ice breakers", where troops were made to stand on a chair and strip off in front of up to 20 colleagues.
The outgoing head of the Armed Forces says it must "square" a "laddish culture" encouraged to take on adversaries with inclusivity, in order to become more effective.
General Sir Nick Carter was appearing at his final Defence Committee hearing before leaving the role.
The military cannot take its "foot off the accelerator" when it came to tackling issues of discrimination, he also said.
Space and Cyber
What will be the military's role in space? UK Space Command launched just little more than a few months ago, formingpart of the Government's plan to increase Britain's ambitions in the domain of space.
Prime Minister Boris Johnson announced in November 2020 a £16.5bn increase in defence spending over the next four years, with a particular focus on cyber and space.
Cyber is a hot topic in relation to military security and countering international threats.
It was announced by Defence Secretary Ben Wallace in October that the UK Government's new National Cyber Force (NCF) will be located in Samlesbury in the north west.
Offensive cyber operations are those which can disrupt hostile state activities, terrorists and criminals threatening the UK's national security – from countering terror plots to conducting military operations.