<Tr> <Td> September 12 </Td> <Td> Alexandre Ribot resigns as Prime Minister of France; he is replaced by Paul Painlevé . </Td> <Td> </Td> </Tr> <Tr> <Td> September 14 </Td> <Td> Russia declared a republic . </Td> <Td> </Td> </Tr> <Tr> <Td> September 20--26 </Td> <Td> Battle of the Menin Road Ridge (Second phase of the Third Battle of Ypres). </Td> <Td> </Td> </Tr> <Tr> <Td> September 21 </Td> <Td> Costa Rica severs relations with Germany . </Td> <Td> </Td> </Tr>

Why did great britain declare war on germany ww1