Tensions between European and Mexican governments have existed for a very, very long time. This was a particularly petty attempt to undermine the Mexican people by the European monarchy.
In 1828, a mob of angry townsfolk destroyed a pastry shop in Mexico City that was owned by a French chef named Remontel. Remontel asked the Mexican government to pay for the damages that the mob caused, but his requests were ignored.
Remontel then asked the French government to help him. They heard his requests at first, but, over time, they forgot about it altogether.
Remontel moved on. Admitting defeat, he used his savings to open another pastry shop. All was well for ten years.
For some reason, King Louis-Phillipe caught wind of the destruction of the pastry shop. He, unbelievably, demanded that the Mexican government pay for the damages.
Not only did King Louis-Phillipe demand that the Mexican government pay Remontel back in full, but they also demanded a 90% interest rate on the 10 years that had passed since.
The Mexican government refused, and the French proceeded to blockade Mexico to occupy the city of Veracruz.
In the end, the war was never really about the pastry shop. The French government only fought the war so they could tax the money that went back to Remontel and claim the 10 years of interest.
French troops remained in Mexico until the government agreed to pay them. The French army pulled out, but the Mexican government staggered their payment, leading to another invasion in 1861. When the French empire fell shortly after that, everyone forgot about the pastry shop.
Well, except Remontel, most likely.