Which side would the conditions favor, and how likely would war be?
The Soviet Union, and pretty near zero. You've got to give someone a motivation for going to war ... a volcano blowing up wouldn't do it. Now, if they did go to war in 1980, the advantage would be with the Warsaw Pact even without the volcano, given the deficiencies in the post-Vietnam U.S. military and the European drawdowns of the 1970s.
Ash what's happening now wouldn't keep aircraft from flying, but it'd keep them from flying high ... which wasn't in the cards for most aircraft by this point, anyhow. SAM coverage and all was pretty good at hitting high-altitude aircraft, hence the deployment of ultra-low-altitude fighter-bombers by NATO.
Now, if you'd said something like 1946, the volcano would have a really big impact on the way that war was fought. OTL, the U.S. was planning to rely on strategic bombing and atomic bombing to balance the Soviet advantage in numbers. The volcano would preclude that by denying high altitudes to American bombers. Good idea for a POD, though.