The French did, from mobilizing on the 26th August they were ready for action on the 7th of September.
The state of Saarland had been occupied by the French from 1920 to 1935, so they knew it well. 11 French divisions advanced towards the town of Saarbrucken and many other salients meeting little resistance, as the Germans had organized an evacuation of the region. However in their wake they had left minefields and traps, dispersed infantry and anti tank weapons.
It's important to keep in mind that the french army more closely resembled a WW1 army at this time, relying on static artillery and dispersed tank support. Their army advanced at a snails pace, with a single machine gun crew holding up a division for an entire day. Despite no German anti tank weapons in the west that could damage the French Char tanks, they crawled along at the pace of their infantry.
This snails pace meant that by the 12th of September only 12 towns and villages had been seized, with the deepest extent of penetration being 5 miles, reaching the Siegfried line which they began shelling. On the same day a meeting occurred between French and British representatives to discuss the state of the war. Warsaw had been encircled on the 8th September but the representatives informed the Poles that a further offensive would continue on the 20th. However the Soviet invasion of Poland on the 17th would spur the French to reconsider. Knowing that their position would be highly exposed when the Wehrmacht and Luftwaffe redeployed. With a drastically reduced time frame before Poland would surrender, French forces withdrew to the Maginot line, leaving their occupied areas in the same fashion they found it, with dispersed infantry, anti tank and mines.
A short campaign with a loss of 2000 men and 4 tanks, ineffective and hence why it is not so well known today.
This campaign has fascinated me however, its my opinion that if the french had behaved like a modern army, and punched their tanks forward as fast and hard as they could. They could have seriously disrupted the course of the war, but the fog of war was thick, and they rightly feared being bombed by the Luftwaffe, not knowing that 90% of it was deployed in Poland.