Russia will want the US to conquer Iran as an excuse to conquer other neighbors like Georgia or asserting further authority over Crimea. China will want the US to conquer Iran as an excuse to conquer Hong Kong and get more leeway with Taiwan. The current Iranian regime has been difficult for Russia to work with meaning Iran getting its ass kicked, at least during the first half of a war, would be beneficial because it forces them to play nice with Russia in the second half of the war. Their economy is so bad and poorly managed that if Japan is anything to go off of, the US remaking their government may actually improve prices of oil and general economic benefit to the world in the long-term. This means any countries such as China benefiting from Iran's oil have something to gain in the long-term.
Trump was nice to Russia for seemingly know reason, and Russia is a mild ally of Iran -that in hindsight justifies Trump's actions because now if Iran wars, Russia won't jump in. Trump kept his mouth relatively shut about Hong Kong -that in hindsight justifies Trump's actions because now if China interferes then Trump can go "… hey, remember how shitty Hong Kong is treated? The US should really fix that." This is not a "Trump was actually a genius all along" theory but an acknowledgement that Trump has, through random chance and intuition, set the perfect stage for a war with Iran.
Thus I theorize that Trump on some level all along has been, whether consciously or unconsciously, uncarefully planning and acting in ways that make it so he can get away with a war with Iran. The worst case scenario for him now is he took out a terrorist, and the best case is he goes to war with Iran and gets exactly what he has been preparing for. Without allies to Iran, with key neighbors Saudi Arabia and Israel happy about the assassination, with the powers Russia and China keeping away, Trump can get away with this with no repercussions. On top of this, Presidents who go to war have a higher chance of winning, which even Trump remarked he thought Obama would war with Iran just to achieve.
He's unabashedly hostile and aggressive toward Iran, pulled out of their nuclear deal, and murdered their top general that Trump himself had designated as a terrorist. From top to bottom Trump has set a perfect stage for himself through these first 3 years of his Presidency through a series of intuitively cohesive acts, and now year 4 will be the climax of the show -the 2020 US-Iran War. This war will be easy for the US to win the first half of and that's the part Trump chiefly cares about. It's the second half -the toppling of the government, potential for Russia to send support to Iran in exchange for significant power, and China intervening out of spite for the US's treatment of their economy, that may prove messy. However, that messy part Trump may just roll with and go "eh whatever" and hand Iran over to those powers, because he got what he wanted. Alternatively, RU and China may genuinely just want to avoid a world war and not touch Iran, in which case Trump gets a flawless victory.
So that's my Iran War Theory. If you enjoyed please give me some KYM Forum Gold.