Well sure but then it relies on the US army. Costa Rica is allied with the US and foolish would be the country trying to declare war to Costa Rica because it'd very probably face the full wrath of Uncle Sam's army.

I'm not saying setting up an alliance with the country that has the biggest army in the world and then doing without an army is a bad thing...

What I'm saying though is that it'd probably be very different for a country without any alliance with the US to decide to have no army.