In 1918 or 1945, local effects (lava or ash) will be not devastating. Yellowstone is not in densely populated region.
But volcanic winter will be extremely severe, badly affecting Northern countries like Sweden or Russia.
If we assume volcanic winter will not cancel the Russian drought of 1946, but superimpose on it..well, the Soviet Union may fall away under pressure of hunger. Or will be less active for several years to come.
But the exact effect of volcanic winter is difficult to predict. May be we will have events as bizarre as skate rinks on Ganges or mass die-off of Malaysians (because they lacked infrastructure to withstand cold spell).
In long term (after ~10 years), eruption would be a big plus for US. The volcanic ash will fertilize their depleted soils of Great Plains, helping to boost domestic agriculture.