All right. Conspiracy time. Mr. X (the eeeeevil bad guy) is a warlock, necromancer, something along those lines. He also happens to be the United States' ambassador to Country Y (something touchy and big - Russia? China?). He has a plot to blow something up using a magical bomb or a magical ritual. He's performing said ritual inside the American Embassy in Country Y. He's being helped by Mr. Q, Mr. R, and Mrs. S, who are - respectively - official representatives of the US, Country Y and Country W (some classic US Ally - the UK?). The players get involved somehow (assignment, goody-two-shoes-itis, whatever) and the story arc climaxes when they interrupt the ritual and attempt to stop Mr. X. Whatever happens, WHATEVER HAPPENS, the blast has to go off - whether it be on its appropriate target, or in the embassy, or whatever... And get some of the assistants killed off. Then, just spin the absolute worst way possible.
Case 1) The blast goes off as planned. Authorities, investigating, find the assistant from the US and/or UK, dead - and involved in the blast. Russia and/or China declare it a major diplomatic incident that descends into war.
Case 2) The blast goes off in the embassy. US authorities investigate - and find the Russian/Chinese agent involved. Major diplomatic incident.
Then just let it spiral out of control. "You $%&(@ performed a terrorist attack on our embassy!" "You're crazy, you were trying to bomb the Kremlin!" etcetera...