The roman engineering achievement that made large public structures possible was

Roman Concrete. Roman Concrete (also called opus caementicium) was one of the most important engineering achievements from the Roman Empire. It was a material based on hydraulic-setting cement, which is what some modern cements (like Portland cement) are based on. There are still some differences that make this concrete special, mainly the fact that they incorporated volcanic ash to prevent cracks from spreading.

i think its provide for the common defense

the monroe doctrine is the best known u.s. policy toward the western hemisphere. buried in a routine annual message delivered to congress by president james monroe in december 1823, the doctrine warns european nations that the united states would not tolerate further colonization or puppet monarchs.