Here is a level cheat for IWAR:

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 IWAR 
from the beginning.