Question

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

Answer 1

How do you make a treaty stick?  Let's offer a few factors.

1.  The treaty needs to be ratified in each nation signing the treaty.  Just because the American president, for example, signs a treaty somewhere doesn't mean it's automatically accepted back home.  Pres. Woodrow Wilson signed the Treaty of Versailles, which included establishing the League of Nations after World War I.  But back in the United States, the Congress rejected American involvement in the League of Nations.

2.  Nations will be motivated to keep treaties their leaders sign because if they don't, the legitimacy of their nations and their leadership will be damaged.  And other countries will see this as an excuse to break whatever other treaties they have with that nation.

3.  There may be international sanctions or other measures that will negatively affect a nation if it breaks away from a treaty.  Diplomats from the other nation might be withdrawn from the country, etc.

Ultimately, treaties are one nation's word to another.  The only ultimate "guarantee" is that nations want their word, their guarantees, to be respected and honored.  Violating treaties they have signed can deeply damage a nation's international reputation.