Considering the destructibility of war equipment, at what point should a country prioritize the preservation of its own resources over the needs of an ally?

If the ally is in need and we have enough tanks here. I don't see the issue with giving them some, if we really feel threatened we can make more tanks to protect ourselves. But as one of the worlds 2 superpowers I don't feel like losing a few tanks is an issue and we can let them stay in Ukraine in it's time of need.

When they are running low on resources and it would cause major damage to their country.

When the echnomy is failing so they should safe guard their own country

it is a need not a weapon now it is used for a tool.