Joe Biden voted for the Iraq War. The story isn’t nearly that simple; he made bipartisan efforts to slow the process down, and he saw his vote more as an effort to give George W. Bush the diplomatic leverage to deal with the presumed WMD issue than a green light to invade. In the end, however, he voted for the war, and it was a mistake. Should it disqualify him in 2020?
In 2008, when the GOP was still dominated by neoconservatives, it surely did. As VP, however, the record shows that Biden was at least as skeptical of interventions as Obama, and argued for smaller American footprints in Iraq and Afghanistan. Besides, the danger now is that the Democratic nominee may move too far in the non-interventionist direction; in a sort of perverse way, Biden’s vote for the war provides some comfort on that point.
And so, when you consider the entire record, as well as Trump’s, the answer is no.