What is the definition of the word betrayed?

To betray someone is to be disloyal to a country, person, group or other organization by acting in such a way to benefit the opposition. You are in effect revealing yourself against the will and desire of someone else. One such famous betrayal occurred in World War II, when Adolf Hitler and Joseph Stalin signed the Nazi-Soviet non-aggression pact, effectively starting the war. Hitler later betrayed Stalin, by breaking the pact and declaring war on the Soviet Union in an attempt to expand German "lebensraum."