<Tr> <Th> Year </Th> <Th> U.S. </Th> <Th> South Vietnam </Th> </Tr> <Tr> <Td> 1956--1959 </Td> <Td> </Td> <Td> n.a. </Td> </Tr> <Tr> <Td> 1960 </Td> <Td> 5 </Td> <Td> 2,223 </Td> </Tr> <Tr> <Td> 1961 </Td> <Td> 16 </Td> <Td> 4,004 </Td> </Tr>

Explain the need for escalation of troops in vietnam