If you are joining the National Guard or the U.S. Army Reserves, you will be protected by federal law from prejudice and mistreatment by any employer. Still, you may not want to give the perception of having misled your employer. That said, you may not want to tell them that you might be gone, perhaps even out of the country, at the discretion of people in Washington D.C. who have pencils behind their ears. And you may not want to mention national emergencies.
As a writer above has recommended, wait until you actually have a date for Basic Training/AIT/weekend drill. Keep in mind that some units will want you to attend drill before you attend your initial training.
Most drills take place on the weekends. Will you be working on the weekends? Is there anyone else at your prospective job who is a weekend warrior?
On the other hand, you may want to inform your prospective employer that you are going to join up. If the training directly applies to your prospective job, that could be a huge feather in your cap. Some military jobs have training which is extremely expensive on the civilian side. Another important factor is whether you will get a security clearance. This can also be of vital interest to a future employer and make you much more valuable.