A Japanese warship has set off to the Indian Ocean to help US ships involved in Afghanistan, days after a law allowing the redeployment was passed.
Japanese ships are returning after a three-month absence
Opposition MPs had tried to block the mission, but Prime Minister Yasuo Fukuda invoked a rarely-used override power to force through the legislation.
The warship Murasame left Yokosuka naval base, south of the capital Tokyo, to meet a supply vessel, the Oumi.
The ships will provide refuelling facilities for US vessels.
Relatives of the Murasame crew and naval personnel gathered to wave the ship off.
"I want you to be proud of yourselves as Japan can now play a responsible role in the international community," the government's top spokesman, Nobutaka Machimura, told the crowd.
"Japan can again join the war on terror, an operation to build peace in the world," he said.
The two ships will arrive in the Indian Ocean in about three weeks.
The Japanese navy has been providing fuel to US-led coalition forces in the Indian Ocean since late 2001.
But the ships were forced to withdraw in November last year after opposition lawmakers - who won control of the upper house in July - blocked an extension of their mandate.
After months of negotiations over the mission, Mr Fukuda invoked his power to override the upper house - the first move of its kind for more than 50 years.
The prime minister says the mission is key to Japan's international image, but the opposition says it lacks a UN mandate and violates the country's pacifist constitution.