Making the annual foreign policy speech at London's Lord Mayor's Easter Banquet, the Foreign Secretary said Britain was ready to support other European countries "vulnerable to the pressure of creeping oligarchisation", a reference to wealthy Kremlin-backed businessmen buying up strategic assets abroad.
His remarks came as Russia declared Ukraine to be on the verge of civil war and as Kiev said an "anti-terrorist operation" was under way.
Mr Hague stressed by annexing Crimea Russia had violated the fundamental principles of sovereignty, territorial integrity and the right of every democratic country to choose its own future.
He said: "We are at a crucial moment in this crisis. Russia must choose whether it is open to diplomacy and de-escalation and, if it decides otherwise, we must be ready for a different state of relations with Russia in the next ten years than in the last twenty."