Britain's three RAF Chinook helicopters and around 50 non-combat troops will be deployed to Mali to provide logistical support to French forces attempting to stabilise the Sahel region of Africa where Islamist extremists have gained a foothold.
The move is seen as trying to ease French anger over a 2003 deal known as the Le Touquet agreement, in which France bears most of the burden in controlling the flow of migrants from the British border in Calais to the UK.
French President Emmanuel Macron and Britain's Prime Minister Theresa May review an honour guard at the Royal Military Academy Sandhurst, west of London on January 18, 2018.
The deal is especially valuable for Britain, because it means that migrants hoping to cross the Channel are stopped at Calais.
But underlying his words was an astonishment that a country, which cleared up on financial services in Europe as a result of the single market created by the eurozone; and one which has acted as a magnet for the most talented citizens of European Union nations, actively wishes to reject these advantages.
Macron also said that Britain could have "no differentiated access to financial services" if it decides to leave the European single market.
Ahead of Thursday's summit, French interior minister Gerard Collomb had called on Britain to pay more towards the costs of dealing with people illegally trying to cross the Channel, as well as accept more refugees and unaccompanied child migrants from France.
Britain and France have an incredibly close partnership in confronting the shared threat of terrorist attacks inspired or directed by so-called Islamic State.
It is Macron's first visit to Britain since assuming office in May a year ago.
Government sources insisted it was not about increasing the number of immigrants coming to Britain.
'What is clear from the discussions we will have today is that a strong relationship between our two countries is in the UK, France and Europe's interests, both now and into the future'.
The leaders announced a new treaty created to ease the suffering of some of the thousands of people camped near the French port - who now wait six months to have their cases settled.
Mr Macron's tough stance underlined his position as probably Britain's staunchest opponent in the Brexit negotiations to come.