When god gives you life, take as many lives as possible
Youjo Senki follows the story of Tanya, a 9 year old girl, in her fight during the first world war... with magic.
This should go without saying, but spoilers ahead...
Tanya wasn't always a 9 year old girl, and she wasn't always in the same universe. Before Tanya was born she was an arsehole of a salaryman working in Tokyo, when one day they fired someone, who pushed them in front of a train. Just before they were killed, god came down to give them a second chance at life, on the basis that it should make them believe in god.
As a result, they are born into a poor family, and soon becomes orphaned, in the universe's Germany, right before the outbreak of war between neighbouring nations.
Tanya is found to have a strong magical ability, and decides to join the army voluntarily, at age 9, instead of waiting to be conscripted.