With tensions between the two sides, war is imminent.
Tips
history
From a Three Kingdoms anecdote recorded in 《魏志》: when Chen Lin was asked why he wrote a scathing declaration against Cao Cao for Yuan Shao, he answered '矢在弦上，不可不发' - 'the arrow was on the string, I could not help but loose it.' Used to excuse actions forced by circumstance.
usage
Nearly always paired with 不得不发 ('cannot but be fired'). Describes momentum or commitment that has passed the point of cancellation.
