The real downside for the rest of the world is without a doubt a second US civil war will result in minimum a few hundred nuclear weapons being detonated. Civil wars are one of the most brutal and nasty forms of warfare where some of the worst crimes against humanity regularly are played out.
Due to the nature of the US nuclear triad all sides of a second US civil war will end up with hundreds of nuclear warheads. Washington state has the US Navy nuclear weapons for the Pacific and Virginia state has the nuclear weapons for the Atlantic. States in the middle of the nation like Montana and the Dakotas have the ICBM nuclear weapons. California and Nebraska have large numbers of US Air Force nuclear weapons with the rest of the Air Force nuclear weapons being kept at dozens of US Air Force bases around the globe. All of the officially inactive but still functional nuclear warheads, numbering in the thousands, are stored in underground vaults at the PanTex facility in Texas.
Not an embassy. Iran has made it clear that should the US push too far their plan is to start blowing up the hundreds of billions of dollars of US industrial infrastructure (oil, refineries, docks, etc) in the Gulf region and North Africa. Most of those state oil companies have their infrastructure owned and operated by US companies like Exxon who then cut them a percentage of the profits.