Theresa May, the woman the United Kingdom has called Prime Minister for the past three years, has been replaced as Conservative Party leader.
In her official resignation statement outside Number 10, she referenced to the ongoing Brexit negotiations and said she had done her best to deliver a Withdrawal Agreement, but acknowledged she had failed.
"It is and will always remain a matter of deep regret to me that I have not been able to deliver Brexit," she said.
After serving as home secretary from 2010 until 2016, Mrs May went to power with an underlying understanding of the defence context.
While being the first British prime minister in four decades who has neither started nor ended a major military operation led by national forces, Theresa May inherited from her predecessor responsibility for combat operations in Iraq and Syria, known as Operation Shader.
Here is a timeline of what she has done for British defence during her term as Prime Minister.
On 18 July 2018, just five days after assuming office, Theresa May opened the debate on the UK's nuclear deterrent.
The debate lasted almost six hours and was followed by a vote during which the Commons agreed to renew Britain’s Trident nuclear programme.
In her first despatch box appearance as Prime Minister, Mrs May warned it would be a "reckless gamble" for the UK to rely on other nations for its nuclear deterrent.
In September 2016, a confidential report on British arms exports to Saudi Arabia was leaked to the media.
BBC Newsnight and the Guardian published the document, which soon led to questions about the relationship between the UK and Saudi Arabia.
While the UK was not directly involved in the Yemeni conflict, Mrs May supported the Saudi Arabian-led intervention in the region.
The outgoing Prime Minister also rejected calls coming from MPs from business, defence, foreign affairs and international development committees to stop arms trade with Saudi Arabia.
She also suggested that it was important for the UK to keep on entertaining relationships with Saudi Arabia because of common objectives in the fight against terror.
The same month, during the G20 Summit held in Hangzhou, China, Theresa May reportedly spoke to the Saudi Crown Prince to ensure that the allegations of war crimes and human rights violations during the Saudi military intervention were properly investigated.
Soon after Donald Trump’s inauguration as President of the United States, the White House announced that the British Prime Minister would meet the President in a week’s time, suggesting that the relationship between Britain and the US "can always be closer".
On 27 January 2017, Theresa May became the first foreign leader to meet the newly-appointed US President.
During a joint press conference at the White House, both May and Trump reaffirmed their commitment to the “special relationship” between the two countries.
After the meeting, Mrs May remarked that Mr Trump had told her he was "100% behind NATO" and she aimed to follow the same strategy when looking at investment into the Alliance.
The same month, it came to light that a Trident test had malfunctioned in June 2016. The outgoing Prime Minister refused to comment on whether she was aware of the incident when she parliamentary debate took place in July 2016.
The following day, the parliamentary security committee held an emergency meeting to discuss security arrangements on the estate.
Following the attack, Mrs May spoke to US President Donald Trump, German Chancellor Angela Merkel, the now-former French President Francois Hollande and more foreign leaders.
Two months later, on 22 May 2017, a suicide bombing attack happened at the Manchester Arena during a concert by American singer Ariana Grande.
23 people, among which many children who were attending the event, were killed and many more were injured.
The following day, Theresa May travelled to Manchester to meet the Chief Constable of Greater Manchester Police, the Mayor of Greater Manchester and members of the emergency service who were working to help those in need in the aftermath of the attack.
Being the second terrorist attack in two months, the outgoing Prime Minister remarked that the threat levels in the country remained severe.
She also announced that Operation Temperer, a plan to deploy troops to support police officer in key locations following a major terrorist attack, was put into effect for the first time.
Almost 1,000 British troops were deployed to the police in key locations across the country.
The terrorist threat level was raised to "critical", its highest level, until 27 May 2017, when it was lowered to "severe".
On 3 June 2017, another terrorist attack took place in the UK. A terrorist vehicle-ramming and stabbing took place near London Bridge, resulting in 11 deaths and 48 people being injured.
The following morning, the outgoing Prime Minister announced the incident was being treated as an act of terrorism but the terrorist threat level remained "severe".
After chairing a COBRA meeting, she said a four-point plan to tackle terrorism was being put into action and called for tighter internet regulations to limit extremists’ of their “safe space”.
In July 2017 Mrs May attended the G20 Summit in Hamburg, Germany.
Following intense months for the UK battle against terrorism and extremism at the summit, Theresa May said there “must be no safe spaces” for terrorist funding in the global financial system.
She discussed the importance of focusing security attention to track small amounts of money used to finance attacks and the risk of foreign fighters from Iraq and Syria.
The morning of 15 September 2017, an explosion took place on a District Line London Underground service at Parsons Green station.
While nobody died, 30 people were injured and the attack was claimed by so-called Islamic State and treated as a terror attack.
For the first time since the Manchester Arena attack in May, the national terrorist threat level was raised again to its highest state of “critical”.
For the second time in the year, Operation Temperer was put into action.
Two days later the terrorist threat level went back to "severe".
In November of the same year, Theresa May condemned the actions taken of the Myanmar Army and police forces against the Rohingya Muslim minority.
She compared the actions taken by the Myanmar forces to "ethnic cleansing".
The same month, Mrs May became the first major foreign leader to visit Iraq since Mosul was reclaimed from the Islamic State.
During her visit to the country, she met British troops and the Iraqi forces they are training.
Ahead of the 2018 Invictus Games in Sydney, the outgoing Prime Minister met the team of 72 competitors selected to represent the UK at the event.
On 1 April, she attended the RAF100 Celebration Ceremony at Westminster Abbey.
In May 2018, the Turkish President Recap Tayyip Erdoğan visited the UK and met the Prime Minister.
Following the meeting, Theresa May said Britain is a "true friend" of Turkey, but added that it was important for Turkey not to “lose sight of the values it is seeking to defend”, hinting at the failed coup of 2016.
In June of the same year, concerns were raised from defence chiefs about the budget for the Armed Forces.
During a visit to the UK, Head of NATO Jens Stoltenberg called on Britain to continue investing in defence. He said he expected the UK to continue playing a major international role.
This followed reports that the outgoing Prime Minister had asked then-Defence Secretary Gavin Williamson to justify the country’s status as a Tier One military power.
At the end of June, she attended the 2018 Armed Forces Day.
In July 2018, Mrs May attended her second NATO Summit in Brussels, Belgium (the first had been in May 2018) and only a couple of weeks later, in August, it was announced that Britain would remain a Tier One military power.
August 2018 was also the month when Theresa May visited Africa for the first time as Prime Minister.
During her visit, she met British troops stationed in Kenya and watched the security forces carry out a mock bomb disposal exercise as she learned how highly-trained soldiers dismantle deadly devices.
After drafting a Withdrawal Agreement with the European Union, Mrs May failed to reach an agreement within Parliament at the end of the year.
The uncertainty of the situation and the possibility of a no-deal Brexit on 29 March 2019 activated a series of emergency no-deal Brexit contingency plans.
The contingency plans to be implemented in case of a no-deal in March 2019 included holding 3,500 troops "at readiness" in the event of a no-deal Brexit.
In January 2019, the outgoing Prime Minister was still trying to pass the Withdrawal Agreement in Parliament. However, with Brexit date still set for 29 March 2019, a no-deal scenario was possible at the time.
For said reasons, in a ministerial statement, it was announced that reservists in the British military could be called out on operations in case of a no-deal Brexit.
The move was meant to enable commanding officers to send 'call out' notices to personnel if requested by government departments.
On 23 April 2019, during a National Security Council (NSC) meeting – the contents of which will then be leaked to the press and will ultimately result in the sacking of Gavin Williamson as Defence Secretary.
Theresa May agreed to let the much-criticised Chinese tech brand Huawei develop 5G networks in the UK regardless of security concerns raised by the NSC.
Cybersecurity concerns about her decision were also raised by the United States.
In May, several months after it was announced that one former British soldier will face charges over the Bloody Sunday shootings, it was reported that Mrs May personally blocked ministers from proposing new legislation that could have protected veterans from prosecution for alleged offences during the Troubles in Northern Ireland.
In an official memo, sent on behalf of the outgoing Prime Minister, it set out instructions that unsolved murders during the Troubles "should not contain" proposals for a statute of limitations on historic prosecutions of veterans.
The announcement led to much criticism from veterans within Parliament, like Conservative MP Bob Stewart, who said to be "ashamed" that successive governments had been "complicit" in the pursuit of veterans.
Conservative MP Johnny Mercer also said he would not support the Government in the Commons unless the historical prosecutions of ex-servicemen and women end.
With building tensions in the Gulf of Oman with Iran, Theresa May also had to chair an emergency COBRA meeting and has been discussing the maintenance of the security of shipping in the region.
Mrs May has also been behind the recent plan to deploy 250 personnel to Gao, Eastern Mali, as instability increases in the Sahel region.
Troops will deploy to Mali for three years as part of a 12,500 strong international force.