There is a constant threat of invasion from neighboring countries.

threat 是名词，意思是"威胁、危险"，指可能造成伤害或不利的事物或情况。这个词来自古英语，与"压迫、强迫"的概念相关。在句中指"邻国可能发动入侵"这种潜在的危险状态。