The Liberal Democrats are promising to meet the NATO target of spending two per cent of GDP on defence, if elected into power.
The announcement follows similar commitments from both the Conservatives and Labour over the alliance.
At the same time, the Lib Dems said they would also bring in a "careers for heroes" scheme to fund further or higher education for service personnel who have served for 12 years or more and want to pursue a new career outside the military.
"Liberal Democrats believe in a strong, credible UK armed forces which is properly equipped and trained for the challenges and threats we face in the 21st Century," Mr Farron said.
Meanwhile, former Lib Dem leader Lord Ashdown launched a petition to stop further cuts to the Royal Marines as part of the party's efforts to underline its commitment to defence.
Lord Ashdown, a former Royal Marine, said: "In an unpredictable age, we need forces that are fast, flexible and mobile -that's what the Royal Marines do at a world-class level.
"But the government has just announced damaging cuts of 200 marines, with no assurance that there won't be more cuts in the future."
Defence Secretary Sir Michael Fallon today defended the government's record on military recruitment, as army numbers sat at 79,000, below the government's pledge of keeping the numbers at 82,000.
Mr Fallon said the government could not "force people to join" the armed forces, and said "I can assure you that we are spending a lot of money on recruiting" in the military.
It also comes as shadow defence secretary Emily Thornberry said Jeremy Corbyn now accepts the Labour party's support for NATO, and has now accepted the will of the party that the transatlantic group was an essential part of the UK's foreign policy.