Imagine your OTP meeting when they were young. They grow up together and fall in love. Then, right after graduation,they have a huge fight because person A needs to move to a different country. 

Person A goes and leaves person B. Years later, a war breaks out between the countries person A and B live in, and they both join the military. 

They end up meeting on the battle field and have to choose between killing the one they love, or betraying their country. 

Person A shoots Person B in the back. Person A goes by them and shuts their eyes saying "I will meet you soon."