Um, look at a map of Europe, 1914. An invasvion of Ukraine would not be a war of foreign adventure for the Russians.
Until the end of World War I, Ukraine was Russian. Ukraine only exists because Lenin gave up territory to the Central Powers in the Treaty of Brest-Litovsk so the Bolsheviks could focus on consolidating power after the Russian Communist Revolution.
Russians would just be restoring the terriorial integrity of Russia.