war clouds are gathering idiom literary (also clouds of war are gathering)
战争阴云;战争的预兆
If you say that war clouds are gathering over a particular country, you mean that a war seems increasingly likely there.
It was the 1930s and war clouds were gathering on the horizon in Europe. 时值20世纪30年代，战争的阴云笼罩在欧洲的地平线上。