User blog comment:IndigoOnIceCream/What if France won the Franco-Prussian War?/@comment-4656717-20120519053229/@comment-32656-20120519230436

Debatable, but I'd guess the British alliance. Better supply situation, likely more of the small states on their side - and I suspect the Americans would join them eventually.

Spark, likely the Balkans, or the French-Prussian border. Doubt it would be the Russians and Ottomans that start it.