A £35m boost for mental health care for British veterans and Afghan personnel now living in the UK would be offered by a future Labour government, according to the Shadow Defence Secretary.
John Healey made the pledge as he claimed his party also would seek to make the UK "democracy's most reliable ally".
Preventing climate conflict would be a top priority for the national security strategy while moves to reduce British Army numbers would be frozen, reviewed and reversed "if needed", Mr Healey added.
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Addressing delegates at the Labour Party conference in Brighton, Mr Healey praised British troops for their efforts to evacuate thousands of people from Afghanistan as the Taliban swept to power last month.
He said: "The crisis has reopened searing memories for many who served in Afghanistan.
"Combat Stress says calls to their helpline doubled last month, yet the Government is spending just £20m this year on veterans' mental health.
"So, I want to announce today that Labour would boost this by £35m, with a special fund to support mental health care for British veterans and the Afghan personnel now with us in the UK."
Watch: Defence Secretary Ben Wallace announcing in March that Army numbers will be cut.
On the party's wider ambitions, Mr Healey said he wanted Britain to "no longer be half-hearted about essential alliances and treaties" in the United Nations, NATO, Five Eyes and the International Court of Justice.
He explained: "We will give the highest priority to security in Europe, North Atlantic and Arctic, pursuing new defence co-operation with European NATO neighbours.
"We will lead moves in the UN to negotiate new multilateral arms controls and rules of conflict for space, cyber and AI.
"We will insist on the UK's say with the US as our most essential ally, stepping up Britain's leadership in Nato.
"We will make preventing climate conflict a top priority for our national security strategy and international action.
"We will freeze Tory cuts to the Army, then review and reverse the numbers if needed."
Mr Healey added: "We will make it fundamental that British defence investment is directed first to British industry.
"We will design in Britain, build in Britain, maintain in Britain with a higher bar set for any decisions to buy from abroad."