Yesterday I stumbled across a obscure experiment in open borders.  Under the 1954 Geneva Accords, the Vietnamese were explicitly given 300 days to freely migrate between the Communist north and the non-Communist south.  As Wikipedia explains:

The agreements allowed a 300-day period of grace, ending on May 18,
1955, in which people could move freely between the two Vietnams before
the border was sealed.

The result was predictable: A mass exodus south from the Communist north, a tiny trickle north from the non-Communist south.  In fact, virtually the only people who went north were guerrillas who otherwise would have been trapped behind enemy lines.  Precise estimates vary, but only slightly.  The article on Operation Passage to Freedom states:

Between 600,000 and one million northerners moved south, while between 14,000 and 45,000 civilians and approximately 100,000 Viet Minh fighters moved in the opposite direction.

The article on the 1954 Geneva Conference gives slightly different figures:

1,000,000 North Vietnamese, mostly Catholic, moved south of the Accords-mandated ceasefire line… At the same time, 52,000 people from the South went North.
Communist fighters were urged to remain in the South in case the
election did not go their way.

South Vietnamese leader Diem never saw it coming:

Despite claiming that his northern compatriots had been “enslaved,” Diem expected no more than 10,000 refugees.

The migration magnitudes are particularly striking because the Communists broke the agreement when they could get away with it.  The ports of Hanoi and Haiphong were still controlled by the French; refugees who made it there could escape.  But many weren’t so conveniently situated:

As the American and
French military personnel were only present in the major cities and at
air bases and on the waterfront, the communists tried to stop people
from trying to leave through a military presence in the ruralside to
interdict the flow of would-be refugees...
In parts of the Red River Delta, ferry services and other water traffic
were shut down so that refugees would not be able to travel to
In some cases there were reports of thousands-strong groups of refugees
being forced back by similar numbers of armed communist cadres. As a result, many refugees headed directly for the nearest coastal point to wait for passing vessels…
In some rural coastal areas where it was common for refugees to
converge before boarding vessels to connect to the long-distance naval
vessels taking them south, the Viet Minh installed mortars on the
beaches to deter prospective immigrants.

They prohibited mass gatherings in an attempt to stop entire villages
or other large groups of people from emigrating together, and also
isolated people who sold their water buffalo and other belongings, as
this was a clear sign that they intended to end their farming. Both the Americans and the South Vietnamese lodged complaints to the International Control Commission about the violations of the Geneva Accords, but little action was taken.

Credit where credit is due: the American navy and the French military were the main rescuers: 310,000 and about 500,000 respectively.  Blame where blame is due: If the American government had learned from this experience, it would have opened its borders to the victims of Vietnamese Communism as it withdrew from the war.  Not only would this have averted most of the post-war Indochinese bloodbath.  It would have been a crushing moral victory over Communist tyranny.  Alas.