soo͞″pər-sēd′
transitive verb
To take the place of; replace or supplant.
Obviously, the millions of illegals who have, and are, being brought here without the consent or permission of legal American citizens supersedes concerns about foreign countries (emphasis supplied)
As important as maintaining our southern border is for the reasons you expressed and more, why should concern for our southern border "replace" our interest in defending Ukraine's sovereignty which certainly includes defending Ukraine's border?
Are we incapable of doing both at the same time? Does the failure to enforce our own border mean that it is not desirable to defend Ukraine's ? Does it mean we do not have the capacity? Would a commitment to defend our own border compel us to defend Ukraine's border? If we were to defend our own border with that furnish the capability to defend Ukraine's?
These are foolish and illogical false choices. The decision on one does not "replace" the other. Each stands on its own.