I wonder who would side with a country running out of resources? they have nothing more to offer, they would be the ones easier to conquer.
the issue tends to be that even if a nation known for its mines is running out of resources, they still have the foundation built upon those resources to work with. lots of experienced, skilled smiths and a well equipped military.
mineral nations tend to end up being the ones who excel in their weaponry and armaments and by extension more likely to succeed in the conquest of new territory. afterall the first nation to be capable of forging weaponry from a new, more capable mineral ends up having a massive advantage during a conflict.
when the conflict is between them and a "prosperous but not powerful" nation, onlookers don't even need to side with those trying to conquer new territory, just by not aiding the otherside they can claim a certain degree of "merit" and a slice of the spoils. the only thing better than have a prosperous ally is being able to claim that prosperity for yourself.
and if the invaders disagree? well they just exhausted themselves in a war and no longer have the capacity to defend themselves if they face get invaded in turn.