...make that the worst environmental disaster in US history.
Despite being in the gulf (and quite huge), Ixtoc 1 was in the Yucatan, more than 500 miles from from US shores. It's impact on US estuaries was far less due to to both dilution and the two months of warning in which there was time to form and implement a mitigation plan. This blunted the environmental effects in the US considerably. Although the damage in Texas was still significant, it's was not nearly what is now expected in LA/MI/AL (no matter what measures are taken.) The barrier islands protecting the TX coast are much more substantial & continous than those in LA/MI too- it was simply easier to protect.
As a single-event type of environmental disaster, it's hard to think of any past incidents in the US that will have the kind of impact that this oil spill will. Killing the grasses that hold the barrier islands together will leave the the inner coastal marshlands set up for the 1-2 punch on gulf storms in years to come. If the coral reefs in FL are impacted things could get very dicey in the Keys over the next few decades too. The Florida Keys are much closer to this spill than Texas is to Ixpoc-I. This party is just getting started.