The compatriots on both shores are separated by just a narrow strait and bound by blood.
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From 《南史·陈后主纪》: Emperor Wen of Sui, preparing to invade the Chen south of the Yangtze, said 我为百姓父母,岂可一衣带水不拯之乎 — as father and mother of the people, how can I leave them unsaved across a mere belt of water? The phrase began as a pretext for invasion; now it almost always denotes friendly closeness.
register
Diplomatic register. Standard in news headlines and speeches about neighboring countries (especially China-Japan, China-Korea, cross-strait). Positive, peace-oriented connotation in modern use.