<Tr> <Td> British guarantee to Poland </Td> <Td> Mar. 1939 </Td> </Tr> <Tr> <Td> Invasion of Albania </Td> <Td> Apr. 1939 </Td> </Tr> <Tr> <Td> German ultimatum to Rumania </Td> <Td> Apr. 1939 </Td> </Tr> <Tr> <Td> Soviet--British--French Moscow negotiations </Td> <Td> Apr.--Aug. 1939 </Td> </Tr>

Why did germany and the soviet union signed a pact