What if Russia cuts undersea optic cables?

Yes, and maybe more than the Russians should be of us taking the same sort of retaliatory action against them. The reason being that while we think if the internet as being global it is in reality somewhat local and the Russian internet is more self contained and based in a geographically contiguous region that the US internet or European internet that relies on servers on both sides of the Atlantic. If the NATO allies cut Russia's access tto the European internet the effect in Russian citizens and the Russian State would probably be fairly minimal. If Russia cut Europe's access to the USA, the effects would be greater.

Then we cut every cross connect going to a Russian ISP and telecom at DE-CIX Frankfurt, and every eastern border of a NATO state.