求问这个句子的的理解

蓝雪舞UID: 3395566

At a time when many of the institutions that had helped maintain imperial societies were beginning to recede in influence, and when the pressures of administering numerous overseas territories and large new domestic
constituencies mounted, the ruling elites of Europe felt the clear need to project their power backward in
time, giving it a legitimacy that only longevity could impart.
请问一下后面的giving it a legitimacy that only longevity could impart. 的理解？