请问下面句子中为什么用be 动词的原形?

问题描述:

请问下面句子中为什么用be 动词的原形?
句子:I have directed that all measures be taken for our defense.
补充:句子绝对没错--是出自abraham lincoln的演说辞

其实完整的说法是I have directed that all measures(should) be taken for our defense省略了should,should be taken 表示应该采取~