Treaty:

A party's consent to a treaty is invalid if it had been given by an agent or body without power to do so under that state's domestic law. States are reluctant to inquire into the internal affairs and processes of other states, and so a "manifest violation" is required such that it would be "objectively evident to any State dealing with the matter". A strong presumption exists internationally that a head of state has acted within his proper authority. It seems that no treaty has ever actually been invalidated on this provision.[citation needed]

Please answer a question about this article. If the question is unanswerable, say "unanswerable". For what does a strong presumption exist internationally that a head of state has acted within in entering into a treaty?
his proper authority