What was the most destructive volcanic eruption in the history of the United States?

The May 18, 1980 eruption of Mount St. Helens (Washington) was the most destructive in the history of the United States. Novarupta (Katmai) Volcano , Alaska, erupted considerably more material in 1912, but owing to the isolation and sparse population of the region affected, there were no human deaths and little property damage. In contrast, Mount St. Helens' eruption in a matter of hours caused loss of lives and widespread destruction of valuable property, primarily by the debris avalanche, the lateral blast, and the mudflows.

Tilling, R.I., Topinka, L.J., and Swanson, D.A., 1990, Eruptions of Mount St. Helens: Past, Present, and Future​: U.S. Geological Survey General Interest Publication, 56 p.