<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> <Tr> <Td> 1973 </Td> <Td> 68 </Td> <Td> 27,901 </Td> </Tr>

When did north vietnam declare war on south vietnam