While mutual aid systems and agreements vary from area to area and state to state, one thing is clear: most departments would be hard pressed to handle large emergencies without the help of neighboring companies, and some small departments would be almost completely non-functional for anything larger than a trash fire with out first response mutual aid.
That said, what information do you have about the history of mutual aid ?

Where I grew up in Liberty, NY, we had a large fire on friday, June 13, 1913 (wow ... now that's got to be unlucky !) which destroyed half of the village's business district. Accounts states that fire companies from a number of surrounding communities responded to help in fighting the blaze, including some which were transported with their equipment on a special train made up of flatbeds sent out by the NYO&W railroad.
There were no motorized fire apparatus in that area in 1913, so I can only imagine hose carts and horse drawn pumpers loaded in flatbed train cars with firefighters hanging all over them.
I am also aware of other large fires in the area at that time which brought a similar response, and have read accounts of mutual aid in other area of the northeast, but is there anyone out there who has their own stories of mutual aid at a time when there were no agreements or organized systems ?