Original: By the war's end, Austria-Hungary and the Russian Empire had collapsed and disintegrated; Bessarabia, Bukovina and Transylvania proclaimed unions with the Kingdom of Romania in 1918.
Simplified: By the time World War I was over, Austria-Hungary and the Russian Empire had fallen apart. This allowed Bessarabia, Bukovina and Transylvania to become part of the Kingdom of Romania again in 1918.