Some basic concepts:

1) Policy in a democracy depends on legal authority.
2) A border marks where the power of one state to make, impose and implement policy and law ends and that of another begins.
3) Borders may be arbitrary lines on a map, but they are not meaningless.

4) Borders are permeable unless closed.
5) If you cannot close a border, you neither control what happens on the other side nor can you prevent movement across it, legal or otherwise.
6) To close a land border requires geography or the coercive power of a state.

7) Coercive power of states is finite. To move it to a border requires moving resources of coercive power from elsewhere, leaving gaps there.
8) The coercive power of a state runs only within that state, not on the other side of a border.

9) If a key part of a desired policy is a closed border, you must either close that border on your side, obtain agreement from both sides to close it, or accept the key part of the policy is missing.

10) That something would be desirable to assist in public health does not mean it can work by the unilateral exercise of state power where a key element is a closed border.
11) One should not let the perfect be the enemy of the good, but the possible trumps the desirable.

Now, you can caveat a lot of those, and develop them further, but there are limits to what even Leviathan can do, and the range of practical options to make another state do what the right thing are finite, too.