Austria was one of France's northern neighbors.*
* Well - apart from the countries in between, of course. I've seen Abstergo mess with history plenty of times. But geography is a new one. (Austria did control what was to become Belgium at the time, so perhaps that's what was meant, but come now, people, clarity!)
And their history leading up to the revolution was notoriously rocky.
Even the King's marriage to an Austrian woman didn't smooth relations between the countries - Marie Antoinette was regarded with suspicion, and gossip accused her of having too great an influence on the King's decisions.
In 1792, the Legislate Assembly decided that war was a good way to export its problems** and declared war on Austria. The war progressed slowly until a mob attacked the Tuileries Palace in Paris, and the Austrians began to invade France. They easily took Verdun - a short march from Paris - causing a panic in the city, and probably leading to the September Massacres.
** Only they called it "spreading the revolution". A wonderful political spin.
The French troops however, rallied to form a resistance, and held off the Austrians at Valmy on September 20. The battle was trumpeted as a major victory for France and resulted in the beginning of an Austrian retreat.