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China and India: two massive countries with huge populations that share a very long border but are not politically or culturally aligned. It wouldn't be surprising if these two giants had fought each other in several wars, but in fact, apart from one month-long battle in 1962, they never have. Why? Because between them is the highest mountain range in the world, and it is practically impossible to advance large military columns through or over the Himalayas. As technology becomes more sophisticated, of course, ways are emerging of overcoming this obstacle, but the physical barrier remains a deterrent, and so both countries focus their foreign policy on other regions while keeping a wary eye on each other.