The geometric metaphor does not quite tell the story here: Lines either are parallel or they are not, “not exactly parallel” being synonymous with “not parallel.” Looked at another way, the Yamamoto comparison emphasizes what is most troubling about the Soleimani assassination, which is, I think, the correct word. The United States was in a state of declared war with Japan, and targeting a senior military leader was a military operation with a military purpose. We are not in a state of war with Iran, declared or otherwise, and the killing of Soleimani was directed at political rather than military ends: It is not part of a military effort to vanquish the Iranian armed forces but part of a political project, the goal of which is to make such a military campaign unnecessary by terrifying Tehran into changing its policies.
Congress can declare war on Iran, as the Constitution empowers it to do, as it did in the case of Japan prior to the Yamamoto operation. Or it can decline to do so, which has been the case so far and which does not seem likely to change. The argument that this act of war against Iran is licensed by an ancient congressional authorization of force against the government of Saddam Hussein in Iraq is not especially strong. “If Congress won’t act, I will” was bad policy in the Obama administration, and it is bad policy in the Trump administration.