enlist
To join the military by choosing to sign up.
Enlist means to join the military voluntarily. When someone enlists in the Army, Navy, Air Force, Marines, or Coast Guard, they sign up to serve their country as a soldier, sailor, or other service member. Unlike being drafted (forced to join), people who enlist make the choice themselves.
When you enlist, you're putting yourself on a list of people ready to serve.
Beyond the military, enlist can mean recruiting someone's help for any important task. A student council president might enlist classmates to help organize a school fundraiser. A scientist might enlist volunteers to help with a research project. When you enlist someone's aid, you're asking them to join your effort and contribute their skills or time.
The word suggests serious commitment and meaningful contribution. When you enlist help, you're inviting people to commit to something important and contribute sustained effort. You might ask your friend to pass the salt, but you'd enlist their help to build an elaborate science fair project.