The killing of top Iranian general Qasem Soleimani in Iraq on the orders of U.S. President Donald Trump has intensified regional uncertainty, sparked fears of rapid escalation and left many questions unanswered.

"I think that the question that we ought to focus on is, why now?" presidential hopeful Elizabeth Warren told CNN on Sunday. "Why not a month ago, why not a month from now?"

To many observers the kill order was a shock tactic devoid of a strategy. Skeptical U.S. politicians and analysts have been at pains to describe the Iranian as a "bad actor" while also querying the reasoning behind the killing as well as its legality.

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Soleimani had been in U.S. sights before – presidents George W. Bush and Barack Obama both opted not to target him, former officials have said – his operational background was widely known and Trump has previously stepped back from taking direct military action against Iran.

So, why now? There have been several answers from the Trump administration, the most consistent being the claim Iran's powerful military official was plotting "imminent" attacks that would target Americans.

Legal experts differ on whether the killing was lawful, but the imminence of any threat is critical in justifying the act as self-defense. What "imminent" means in this case remains unclear.

Chairman of the Joint Chiefs of Staff Mark Milley has said Soleimani planned a "significant campaign of violence" against the U.S. in the coming days, weeks or months.