A war with Iran. He will tear up the Iran deal. Iran will see this that you cannot deal with the US in good faith and restart their nuclear program. Trump will try sanctions but it won't work because the EU,UK,China,Japan,Russia all will want to do trade with Iran. Then all his anti Iran advisors will tell him to bomb Iran. He agrees with it along with Israel. It back fires since Iran has the S300 missiles.
Iran shoots down Israeli and US jets. Iranian speed boats fire anti ship missiles at American ships in the Persian Gulf. Iran fires ballistic missiles in Saudi Arabia and other Gulf nation bombarding oil facilities raising the price of oil sky high. Mines blow up oil ships. Iran activates their proxies in Saudi Arabia,Bahrain,Yemen to spread chaos in the region.