Why did the founding fathers create a week national government to protect the powers of the states

Examine the causes, course, and consequences of united states westward expansion and its growing diplomatic assertiveness (war of 1812, convention of 1818, adams-onis treaty, missouri compromise, monroe doctrine, trail of tears, texas annexation, manifest destiny, oregon territory, mexican american war/mexican cession, california gold rush, compromise of 1850, kansas nebraska act, gadsden purchase).