Clinton and Russian Foreign Minister Sergey Lavrov will meet on the sidelines of the Munich Security Conference to formally begin the "New START" pact that cuts how many strategic nuclear warheads each country can deploy.
The Senate approved the agreement in December after President Barack Obama pressed strongly for its passage. Russia ratified the deal last week.
Clinton arrives Friday at the conference where world leaders and top diplomats talk policy in an informal setting. She will meet various officials before leaving Sunday.