The Han, Roman and Gupta empires all fell do to invasion. A strong, loyal army is necessary to protect the empire from invasion. All of these empires were first weakened by a succession weak governments. The trust and faith in the government was eroded over time. Internal strife by the people because of declining abilities of the government to govern weaken the state. The government should try to prevent internal rebellions by maintaining good governance over the people.
It was the "Open Door Policy" that was the name of the trading policy that Secretary of State John Hay initiated with China and the European nations that already had a presence there, since the US wanted to benefit from trade with China as well.
