NATO pact

There are 30 countries in NATO and the Ukraine is the only country bordering Russia that is not a member of NATO. NATO has an agreement that if one of the countries are attacked then all are attacked. Russia can attack Ukraine without attacking NATO but that is the only country on their border that is not a NATO member. If Russia takes Ukraine will Putin stop there. If he goes into a NATO country does that mean war between Russia and NATO. Does that mean war between nuclear powers. Will that lead to WW 3? Will western countries believe WW 3 is necessary to protect Slovenia. Will Americans be willing to risk nuclear war to protect the people of Slovenia, a country most Americans never heard of and certainly have no idea where it is. France, England and Poland had a mutual protect agreement and when Hitler went into Poland that started WW 2. Will history repeat?