First Lt. Ehren Watada's supporters -- including clergy and a military family group -- said he is the first commissioned officer to publicly refuse to serve in Iraq and risked being court-martialed.
The Pentagon said Watada was among a number of officers and enlisted personnel who have applied for conscientious objector status.
"The wholesale slaughter and mistreatment of the Iraqi people is not only a terrible moral injustice but a contradiction of the Army's own law of land warfare. My participation would make me party to war crimes," said Watada in a taped statement played at a Tacoma news conference.
His superiors at the nearby Fort Lewis military base would not let Watada leave the base to attend the press conference. Another news conference took place in Watada's native Hawaii.
Watada, 28, had been scheduled to be deployed to Iraq for his first tour later this month. He joined the Army in 2003, and has served in Korea.
Watada said his moral and legal obligations were to the U.S. Constitution "not those who would issue unlawful orders."
Nearly 2,500 U.S. soldiers and an estimated 40,000 Iraqi civilians have been killed since the U.S.-led invasion of Iraq in March 2003.
In recent weeks, Marines have been accused of killing 24 Iraqi civilians in the town of Haditha, raising concerns about abuse of force.
Paul Boyce, Army spokesman at the Pentagon, said Watada's case was being reviewed, adding it "is not the first case, nor is his case particularly unique."
Joe Colgan, whose son Benjamin was killed in Iraq, said sending sons and daughters to Iraq was "unpatriotic."
"I ask that we all think about our moral conscience and what we have done in God's name," said Colgan.