When two countries sign a treaty, what guarantees it will be upheld?

A treaty is an international law agreement signed between actors on the international law (mainly sovereign nations and international organizations).

When two countries sign a treaty, the execution of that is stated to be interpreted "in good faith" by the Vienna Convention.

However, if a modern treaty is signed between two countries and it is registered within the United Nation it is possible to appeal to the International Court of Justice, which is the judiciary organ of the United Nation.