Cheat Code:

Open the IWAR directory on your hard drive. Inside there should be a folder named "PSG".
Open it. Inside of the PSG folder there is a folder named "RESOURCE".
Open this too. Inside of the "RESOURCE" folder there is a folder named "SCRIPTS".
Inside of "SCRIPTS" there should be a folder named "PRECEDENCE".
Inside of the "PRECEDENCE" folder there should be a file named "PREC" which you should open with a spreadsheet program, like Excel, and change all the numbers in the "PREC" column to 0, and save.

Now you can access all the levels of I-WAR from the beginning.