For the military veteran reader, often most agencies will not discuss hiring without your DD Form 214, or discharge from the service. An agency does not want to commit to a process and then have your tour of duty get extended or whatever. Also, the DD Form 214 has coding on it that will give great insight into your past career and skills. It is often a ticket to the next step.
Yes, there are some departments that will allow you to apply and begin the process without it, but having the 214 will move you along in the process. Be sure to talk in depth to the recruiter or human resources office about this.
Some departments require an initial application. Then if you meet the minimum requirements, they will require a more in-depth application to assist with the background investigation and polygraph. Some departments may want it all up front at the beginning.
Whatever you do, if you can, make a copy of what you submitted. If you are asked about your past by a background investigator and you give different answers, this can be a problem. I did mention backgrounds and polygraphs, if you have done something in your past that can be a cloud on the horizon, find out what the disqualifiers are now to save future embarrassment.
Face it, if you have had youthful misadventures and indiscretions, better to declare it now; it shows integrity. Yes, it could eliminate you from consideration, but be up front first. Don't let them find out about it on their own. Yes, I have seen police applicants arrested by the background investigators for failing to disclose vital information.