Why does NATO has to defend Lithuania

NATO has to defend Lithuania, Estonia, Latvia, Bulgaria, and Romania. If Russia decided to start a war in the eastern part of Europe, NATO is forced to defend these countries. A war with Russia will be devastating for the whole continent. People from these countries are openly calling for war with Russia. Russia has no interest in starting a war with NATO. If NATO dropped its promise to defend the countries in Eastern Europe, Russia would have no reason to attack, and war could be prevented.