During the crisis, decisive leadership was lacking at the highest levels.

was 是 be 动词的过去式，表示"存在"或"处于某种状态"。 lacking 是动词 lack 的现在分词形式，lack 的意思是"缺少、缺乏"。这里 was lacking 构成过去进行时，表示"当时正在/持续缺乏"的状态。