勿忘国耻

wù wàng guó chǐ

Never forget national humiliation, refers to Mukden railway incident of 18th September 1931 九一八事變|九一八事变 and subsequent Japanese annexation of Manchuria

勿

wù

do not

忘

wàng

to forget

国

guó

country; nation; state

耻

chǐ

(bound form) shame; humiliation; disgrace