According to the Illinois Marriage and Dissolution of Marriage Act (750 ILCS 5), any defendant in a lawsuit must be notified of pending legal action. This includes your spouse if you are filing for divorce. Even if your spouse is serving overseas, he or she still must be served with the divorce papers. The first step will be to contact the authorities at the base where your spouse is stationed, and determine what official channels can be used to serve divorce papers.
Of special note, you should be aware that the Soldiers and Sailors Civil Relief Act, 50 UCS section 521, protects active service members from being found in default for failing to respond to a legal action. This means that your Illinois state divorce proceedings can be delayed until 60 days after your spouse completes his or her tour of duty.