<Tr> <Td> 1969 </Td> <Td> 11,780 </Td> <Td> 21,833 </Td> </Tr> <Tr> <Td> 1970 </Td> <Td> 6,173 </Td> <Td> 23,346 </Td> </Tr> <Tr> <Td> 1971 </Td> <Td> 2,414 </Td> <Td> 22,738 </Td> </Tr> <Tr> <Td> 1972 </Td> <Td> 759 </Td> <Td> 39,587 </Td> </Tr>

What effect did the withdrawal of american troops have on south vietnam