mission creep
noun [ U ] disapproving uk/ˈmɪʃ.ən ˌkriːp/ us/ˈmɪʃ.ən ˌkriːp/
（尤指军事行动中的）使命偏离，任务蠕升
the fact of doing a much larger job for a longer time than was originally expected, especially in a military operation
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