Holbrooke, who has served as US ambassador to the United Nations and as the top US diplomat for Europe and for East Asia, said it would likely fall to the next US president to decide whether and how to extricate America from the conflict.
Regarded as a potential secretary of state if the Democrats win the White House in 2008, Holbrooke delivered an unsparing assessment of US President George W Bush's foreign policy during a lecture on US-Turkish relations.
''On every side Turkey faces challenges of enormous proportions,'' he said. ''To the south and southeast, neighbors called Iran and Iraq -- one a dangerous, destabilising religious dictatorship, the other a civil war raging out of control.'' ''Iraq already presents us with the worst situation internationally in modern American history. Worse even than Vietnam,'' Holbrooke added, noting he served in Saigon and worked on Vietnam in Washington and at the Paris peace talks.
''I never thought I would say anything was worse than Vietnam but Iraq, my friends, is worse than Vietnam,'' he said, voicing hope that the current US offensive will succeed but saying the chances of this ''are not high.'' Following Bush's January announcement of a ''surge'' in US troop levels in Iraq, the Pentagon is increasing US force levels in Iraq by about 30,000 troops in an attempt to regain control of security and to reduce sectarian violence.
There are about 146,000 US troops in Iraq more than four years after the March 2003 US-led invasion that toppled former Iraqi dictator Saddam Hussein.
''If it does not succeed, then the United States will face an even more difficult set of choices,'' he added, describing these as increasing troop levels, holding them steady, trying to disengage from ''the battle of Baghdad'' while fighting al Qaeda elsewhere in Iraq, or simply withdrawing.
''You can assume, safely, the current administration will reject the last option ... and look for ways to salvage something from the wreckage of its own misguided policies,'' he added.
As a result, he said Bush would bequeath his successor the war in Iraq, the continued US presence in Afghanistan, where about 26,000 US troops confront a renewed Taliban insurgency, as well as the standoff with Iran over its nuclear program.
''We must assume ... that the next president will inherit the most difficult foreign policy challenges ever to land in the Oval Office on day one,'' he said.
''Whoever it is, the odds are very high that disengagement from Iraq -- unless the war is clearly being won -- will be a very high priority,'' he said. ''Any withdrawal ... should not be done precipitously, of course, but no one can predict now what the next president will actually confront.''