This is just a selection of what worries me:
Trump already made a fool of himself at NATO (strange bizarre interviews, pissing contests that pissed world leaders of)
He wanted to invite Al Qaeda to the White House
Met with Erdogan in the White House
Started mess in Syria without the knowledge of NATO (backstabbing Kurds -allies- who helped against ISIS), this resulted in ISIS warriors getting away and this means a big threat also for Europe/terrorism
Froze important aid to Ukraine in the battle against Russia
Asked for a 'favor tho' - which resulted in leaders who are afraid to work with the Americans because all help will be used in a 'see the world needs me' rant by Trump
Leaked confidential phone calls to defend his ass in the corruption scandal
Started a trade war with European countries
Started confrontations with NK and insulting him with nicknames like Rocketman to provoke, also starting a trade war with China which doesn't help because USA needs to work with China against NK
Started mess in Iran without asking advice or communicating this to allies
Banned companies from building the gas pipeline to Russia for Germany, which means the USA is interfering in global trade - USA sanctions against European countries forces Europe to work more closely with China/Russia and to distance EU from Washington
https://www.thelocal.de/20191222/germany-eu-fume-at-us-interference-over-russian-gas-pipeline