What is the ruling on working as a security guard at the American embassy in a Muslim country, and is the guard sinful if he defends it in case of attack, especially given that there is an embassy for the Muslim country in America?

It is permissible to work as a guard for the embassies of disbelieving nations, and to hire oneself out to them for permissible tasks. It is also permissible to defend them if the embassy is attacked, because attacking disbelievers who are under a treaty is not allowed.