<Tr> <Th_colspan="2"> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> Women in militaries </Th> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <Tr> <Td_colspan="2"> <Ul> <Li> Ancient </Li> <Li> Post-classical </Li> <Li> Early modern </Li> <Li> Women in 18th - century warfare </Li> <Li> 1800--1899 </Li> <Li> 1900--1939 </Li> <Li> The World Wars <Ul> <Li> WWI </Li> <Li> WWII </Li> </Ul> </Li> <Li> 1945--1999 </Li> <Li> 2000--present </Li> </Ul> </Td> </Tr> <Ul> <Li> Ancient </Li> <Li> Post-classical </Li> <Li> Early modern </Li> <Li> Women in 18th - century warfare </Li> <Li> 1800--1899 </Li> <Li> 1900--1939 </Li> <Li> The World Wars <Ul> <Li> WWI </Li> <Li> WWII </Li> </Ul> </Li> <Li> 1945--1999 </Li> <Li> 2000--present </Li> </Ul>

What was the critical battle front in ww1