What was the main reason why France go to war in world war 1?

It should be noted that it wasn't just fought in France. It was also fought in:

1. Eastern Europe. Russia fought against Austria-Hungary and Germany. Russia did OK vs Austria (historically, Austria had never really been very good at war, and World War I was no different) but was usually badly defeated by Germany. Russia's poor record partly induced their two 1917 revolutions and Communist takeover.

2. Northern Italy/southern Austria. Italy joined the Allies in the middle of the war and fought their old foe, Austria.

3. The Balkans. The war started when Austria attacked Serbia, and Austria did not quickly defeat Serbia.

4. The Middle East. The Ottoman Empire joined the Central Powers, so the Allies attacked them, famously at Gallipoli (near Istanbul). The British also convinced the Arabs to revolt against the Ottomans and sent armies to help the revolt, leading to fighting there too.

5. Africa. Germany had colonies in Africa, and also had small armies there. The German general Paul von Lettow-Vorbeck famously never lost a battle in his guerrilla war against much larger armies of British, Portuguese and Belgian troops in East Africa around modern day Tanzania.

6. Asia and the South Pacific - Germany had colonies out there too. Japan actually contributed to occupying German territories, since Japan was part of the Allies at that time.

And that's just the land war, and doesn't include the war at sea.

However, the fight in France seems to be more well known than the rest of those campaigns. The fighting in France happened because Germany was trying to knock France out of the war- Germany had declared war on Russia (France's ally), and knew France would come to Russia's aid, so Germany attacked France first. Germany knew that Russia could make very large armies, but because Russia was not very modern, it would take a long time for the armies to get together and then ship off to fight Germany. France, on the other hand, was nearby and very modern, and Germany wanted to defeat them quickly before Russia's large armies could arrive in eastern Germany.

So the plan was to go through Belgium and march to Paris before France could stop them. Belgium refused to let Germany send armies through their territory, so Germany did it anyway (which caused England to join the war), and marched into northern France. However, the plan failed, and the invasion of France bogged down into bloody trench warfare.