Something was unmentionable in polite company gathered to dedicate George W. Bush's presidential library here Thursday: the words "Iraq war."

But memories of the US invasion in 2003 lingered anyway, like an uninvited guest, as living US presidents came to Dallas to honor one of their own.

The gathering of the powerful clan was hardly the place to reargue one of the most divisive issues yet in 21st century US politics.

But the silence was notable nonetheless, because the Iraq war may be the defining political moment for presidents Bush and Barack Obama.

Iraq, barring a democratic turnaround that becomes an example in the Middle East, threatens to permanently stain Bush's legacy following his failure to find the weapons of mass destruction he used to justify war.

Obama, meanwhile, built his entire 2008 campaign on fierce opposition to what he blasted as a "dumb" war and at the end of 2011 fulfilled his core promise to bring all US troops home.

As the party went ahead in Texas, new fears of sectarian warfare stalked Iraq, where 4,400 Americans and tens of thousands of civilians died in a violent decade after 2003.

Another 179 people perished in Iraq in the last three days alone, in attacks and unrest that again highlight the long-term implications of Bush's decision to wage war, and Obama's to complete a full US withdrawal.

Bush said in interviews before the ceremony that he remained "comfortable" with his decisions on Iraq, and his museum makes the case that he had no option but to use force after Saddam Hussein refused to bend to UN resolutions.

In his speech, Bush made an impassioned case that his actions abroad were based on the cause of expanding "freedom."

Freedom is what Bush and top aides say they provided to the Iraqi people, despite botched US management of the post-war period that sparked an insurgency and tortured politics.

"We liberated nations from dictatorship," Bush said, as he honored service members who laid down their lives to keep America safe "and to make other nations free."

Obama, with a delicate task of praising Bush the man, while skipping over their disagreements, chose to concentrate on his predecessor's role after the September 11 attacks in 2001.

The closest he got to Iraq was a gentle reference to their differences over the war and Bush's empathy for those he sent into battle.

"Even as Americans may at times disagree on matters of foreign policy, we share a profound respect and reverence for the men and women of our military and their families," Obama said.

"And we are united in our determination to comfort the families of the fallen and to care for those who wear the uniform of the United States."

Other guests also had a case of "don't mention the war."

Former president Jimmy Carter, who once called the war "unnecessary" and "unjust" chose to praise Bush for fighting HIV and AIDS in Africa.

Across the stage sat the wheelchair-bound 88-year-old George H.W. Bush, whose own presidential legacy was marked by his decision not to go on to Baghdad after ejecting Iraqi forces from Kuwait in 1991.

In the audience sat a cowboy-hatted Dick Cheney, who ran the Gulf War for Bush as defense secretary, but turned hawkish and was a key voice as vice president in the march to war in 2003.

Another guest also had an Iraq war story -- Hillary Clinton.

The former first lady and secretary of state's Senate vote to authorize the invasion emerged as the biggest liability of her 2008 race for the White House against Obama and alienated the Democratic base.

And Bush was not the only one with a tricky Iraq legacy in Dallas.

His old comrade-in-arms Tony Blair sat in a place of honor -- his enduring popularity in the United States in contrast to the fury over the former British prime minister's war record at home.