It is said that at their peak, every citizen of the Old Republic either encountered a Jedi or the results of a Jedi's actions at least once in their lifetime. The Jedi played many roles in the Republic, from policemen, judges or healers to soldiers, generals or space pilots. Sometimes they played more than one or even all these roles.

The beginning of the end started when it was realised that the Jedi where in decline. It was their numbers had diminished, not their power. Over the last few centuries less Jedi had come forward to replace the aged or dead. It eventually came to the attention of the Senate that some systems had not seen a Jedi for several generations. This caused intense debate amongst the leaders of the Republic as to how they could help the Jedi or what to do if they disappeared completely.

This situation eventually split the Senate. Different factions appeared, with different ways to deal with the situation.
On one side stood the Republicans, they believed that the decline of the Jedi was only temporary, that their numbers would increase again. And if they didn't, because of the lessons that they had taught the leaders of the Republic, they would be able to continue without the Jedi.
Against them stood the Separatists. This group believed that the time of the Republic was at an end. That it was time to dissolve the Senate, and let the systems solve their own differences, without interference.
The third group, like the Republicans, was in favour of maintaining the Senate, but believed that the time had come for the Senate to enforce its own decisions. The Imperialists wanted the Senate to keep its position in the Republic and to have its own military force.

It is not known if the decline was the cause of the rise of the Sith, or if the rise of the Sith caused the decline, but it was during this period evil began to creep into the Old Republic.

The friction between the factions increased as the Jedi continued to become less common throughout the Republic. When the pressure became too intense the Separatist faction attempted to withdraw from the Republic. As neither the Republicans or the Imperials could allow systems to leave the Republic, this resulted directly in war.

Next: The Clone Wars