Why did South Vietnam fall to the communists after the Paris Peace accords?

If someone can me out with this that would be grate why did people begin migrating to the northwest in the 1930s? the dust bowl forced people to move. the drought forced people to move. people moved in search of work. all of the above. what caused the united states to enter world war ii? the attack on pearl harbor the need for war supplies great britain asked the united states to join. the united states didn't join world war ii. why were japanese americans relocated during the war? they had deadly diseases. they were threatening to destroy american companies. people thought they might give aid to japan. they were breaking laws. what finally ended world war ii? japan bombed pearl harbor. ammunition ran low and the germans surrendered. the united states pulled its troops out. the united states bombed japan.