Answer:
If a country is a part of NATO and is attacked/invaded then the other members must come to their defense.
Step-by-step explanation:
As I'm sure you're aware, Russia is invading Ukraine with the purpose of obtaining resources.The Ukraine is currently not a part of NATO. However, one of the agreements within NATO is that if a country in the organization is directly attacked or invaded (in the case of the Ukraine currently) other NATO countries must step in and provide defenses. While the US and other NATO countries are currently providing aid to the Ukraine and accepting refugees, they aren't defending them to the extent that they would be if the Ukraine was in NATO, which is why Putin doesn't want them to join.