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 same month, it came to light that a Trident test had malfunctioned in June 2016. The Prime Minister refused to comment on whether she was aware of the incident when she parliamentary debate took place in July 2016.
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 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".
She compared the actions taken by the Myanmar forces to "ethnic cleansing".
The same month, Theresa 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 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.
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, the Prime Minister failed to reach an agreement within Parliament at the end of the year.
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 the Prime Minister 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 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.