Japan's cabinet has approved sending up to 1,000 non-combat troops to Iraq, its biggest deployment since World War II.
There is strong public opposition to the plan
In line with Japan's pacifist constitution, the troops will only be allowed to play a humanitarian role.
But the plan is highly controversial, especially after two Japanese diplomats were killed in Tikrit last month.
Prime Minister Junichiro Koizumi explained the decision by saying Japan had to prove a "trustworthy ally" for the United States.
"We are not going to war," Mr Koizumi stressed.
PACIFISM UNDER THREAT?
Japan's constitution renounces the use of force
This has been stretched to allow self-defence troops
PM Koizumi wants to give Japan even greater powers
"The situation in Iraq is severe. We know it is not necessarily safe. But
our Self-Defence Forces must still fulfil this mission," he said.
Mr Koizumi said that Japan, which was criticised for contributing only money, and not troops, after the 1991 Gulf War, must live up to its international responsibilities.
"America has made many sacrifices to create a viable democracy in Iraq," he said. "Japan must be a trustworthy ally for the United States."
Under the broad plan proposed by Japan's ruling Liberal Democratic Party, troops will go to south-eastern Iraq to restore water services, offer medical
and other humanitarian assistance and help rebuild schools and other infrastructure.
No timeframe has been given. Mr Koizumi has vowed to dispatch the troops as soon as the situation is secure.
Public broadcaster NHK said Tokyo planned to send between 500 and 700 ground troops, armoured vehicles, up to six naval ships, including destroyers, and about eight aircraft, including three C130 transport planes.
Japanese media reports say the army will have portable anti-tank rocket launchers and recoilless guns to protect against possible suicide bomb
Surveys suggest that at least two thirds of the Japanese public are opposed to the plan.
Critics point out that Japan's constitution bars Japan from using force to settle disputes, and that the troops could be drawn into conflict to defend themselves.
A special law allowing the dispatch was passed by Japan's parliament in July, but only under the condition that the troops be sent to a place away from combat.
Incidents such as the suicide bombing at the UN office in Baghdad in August and the killing of the Japanese diplomats on 29 November have underscored the continued instability in Iraq.
Japan's main opposition leader, Naoto Kan, has vowed to try and stymie the government's plan, telling a public rally on Monday that there was still time before the troops go.