What was the monroe doctrine? a 1,912-mile continuous railroad line constructed between 1863 and 1869 that connected the existing eastern u. s. rail network at omaha, nebraska/council bluffs, iowa with the pacific coast at the oakland long wharf on san francisco bay. the economic theory that trade generates wealth and is stimulated by the accumulation of profitable balances, which a government should encourage by means of protectionism. a us policy of opposing european colonialism in our hemisphere beginning in 1823. it stated that further efforts by european nations to take control of any independent state in north or south america would be viewed as "the manifestation of an unfriendly disposition toward the united states." at the same time, it noted that the u. s. would recognize and not interfere with existing european colonies nor meddle in the internal concerns of european countries the 19th-century doctrine or belief that the expansion of the us throughout the american continents was both justified and inevitable.