After World War I, though, the basic territorial pattern of the Balkans was set. Romania, Greece, Bulgaria, and Albania were independent countries and, with some exceptions, occupied their current borders. Today's countries of Serbia, Montenegro, and Macedonia were then part of Yugoslavia--a new country formed in 1918.
verb (used with object), undermined, undermining. 1. to injure or destroy by insidious activity or imperceptible stages, sometimes tending toward a sudden dramatic effect. 2. to attack by indirect, secret, or underhand means; attempt to subvert by stealth. 3. to make an excavation under; dig or tunnel beneath, as a military stronghold.
When you undermine someone's authority, it means to do things that make people lose respect for them. For example, if you point out your boss's mistakes in front of other people, it undermines his or her authority.