国境 refers primarily to the border line or frontier that separates one country from another — think of a physical boundary (国境线). It can also refer to the national territory within those borders, but the most common use is about crossing or defending the border (跨越国境). If you translate 'national territory' back to Chinese, the more standard word is 领土, not 国境, so be careful to use 国境 when you mean 'border' specifically.