The word most often used to start a war is "provocation." Trump has started a "provocation" with the assassination of Soleimani. Soleimani was certainly guilty of masterminding the use of IUD roadside bombs to kill or maim hundreds of American troops, but will his death lessen the killing of innocents around the world?
A disturbing coincidence in how wars start is found in examining the Gulf of Tonkin or USS Maddox Incident prior to the Vietnam War. Another is the Second Iraq War with a provocation of "Weapons of Mass Destruction" and comparing these two examples to President Trump's recent actions.
The Gulf of Tonkin incident involved two events prior to escalating the Vietnamese disagreement into to a war. The first involved the USS Maddox firing 3 shots across the bow of N. Vietnamese torpedo boats as a warning they were too close for comfort. Those 3 shots initiated a retaliatory response of machine gun fire and torpedoes fired. When the action ceased the Maddox had a single bullet hole to the bridge. The N. Vietnamese lost 4 sailors killed, 6 wounded and damage to 3 torpedo boats. The second and final episode was a factious report of North Vietnamese torpedo boats attacking another U.S. ship in the Gulf of Tonkin, much later (2003) found to be erroneous blips with war as a result.
The Second Iraq War started as a result of an unfounded belief that Saddam Hussein was hiding nuclear, biological or chemical weapons. Simply remembered as "Weapons of Mass Destruction." True, Saddam had used chemical weapons on Kurds in his own country and against Iranians in war. When the UN Council demanded that Saddam destroy chemical weapons and discontinue research on biological and nuclear weapons, he complied. Unfortunately, he wasn't believed with war as a result.
Trump's decision to assassinate Soleimani is a result of a belief that Soleimani was intending to increase terrorist actions throughout the world. Was this dangerous decision made with no more proof than suspicions? Eventually someone will come forward and tell us the whole story just like in the previous two examples. The only question is how many lives will be lost until the whole truth is found? In summary, it seems that confrontations often start with a truth followed by an exaggeration leading to major war.