You can't fight on an empty stomach; without proper nourishment you can't do demanding work or perform well.
試験前に何も食べないで行ったら集中できなかった。やっぱり腹が減っては戦は出来ぬね。
I couldn't concentrate when I went to the exam without eating. As expected, you can't fight on an empty stomach.
遠足の日は朝ごはんをしっかり食べて出発しよう。腹が減っては戦は出来ぬから。
On the field trip let's have a good breakfast before leaving. You can't function properly on an empty stomach.
彼は会議の前に軽食を摂り、腹が減っては戦は出来ぬと冗談めかして言った。
He had a snack before the meeting and jokingly said, 'You can't fight on an empty stomach.'