Which they were forbidden to have under the terms of their surrender in '91 and under U.N. resolutions. Note they were not merely forbidden to have ready-to-use WMD, they were for forbidden to have WMD programs, and they were required to disclose full information about such programs, which we now have manifest proof (the mobile labs) they did not do.
In fact, Saddam's failure to fully disclose WMD programs provided fully adequate legal justification for military action as long ago as last December, when the U.N. inspectors affirmed that Iraq's report on WMD programs, required under resolution 1441, was innacurate and incomplete. This legal basis does not evaporate retrospectively just because Saddam decided (if he did) to dump his bio/chem agents into the Euphrates, or ship them off to Iran and Syria, or whatever, at the 9th hour.
Heck, even if Saddam never had usable WMD, but still for some nutty reason insisted on keeping ineffectual programs hidden from inspectors at the risk of his regime, that still wouldn't effect the legal basis for the war: his utter failure to comply with obligations he accepted to preserve his regime 12 years ago.
But we knew that already since we had supplied them with a bunch of biological weapons when they were fighting Iran.
Is it your usual practice as a FReeper to recycle left-wing agitprop? There is not a shred of evidence, nor are there even credible or coherent accusations that the United States ever supplied Iraq with biological weapons. IIRC, the most that has been asserted (with what actual evidence, if any, I don't know) is that the United States government looked the other way, allowing Iraq to acquire standard laboratory strains of pathogens, and/or precursor chemicals, which they then developed into bio and/or chem weapons.