All leases are different and all states have slightly different landlord-tenant laws.
Some leases allow you to give proper notice, pay a fee and you can leave early. Some leases allow you to sublease. Landlords are obligated to find another lessee as soon as possible, but you are still liable for the rent up until the new tenant moves in. Other leases let you cancel the lease if your employer imposes a mandatory transfer for XX number of miles.
For military personnel, some leases allow you to break it if you receive an order for a permanent change of station or a temporary change for 3 months. However, you have to have been in the military when the lease was signed. I am still looking for a law in any state that gives you this option for joining the military. I would hope that you join the military because you want to serve your country, not because you want to break your current lease.