Contract law and international arbitration do exist though. The ultimate inability to enforce seems like it is a separate issue and not limited to bilateral contracts - nation states are more powerful than international courts in many cases.
The contract was to be enforced by the militaries of these belligerents, and to be interpreted and settled privately.
It may well be the case that this was never intended as a formal contract, but I am responding to the assertion that there was no way for it to be. I stand by the claim: the issue is authority/power, not the necessary legal means.