the.com/losing battle

means A struggle or effort that is already doomed to fail, no matter how hard you keep trying.

from A straightforward military phrase — a 'battle' being 'lost' — extended by the 1700s/1800s into figurative use for any hopeless struggle, like arguing with a toddler or fighting weeds in a garden. No hidden twist here: it's war imagery doing exactly what it says, borrowed for everyday defeats too small for actual armies.