The lease is valid as between the tenant who signed and the landlords. But they are not allowed to simply demand the rest of the year's rent. They must seek other replacement renters and give you a credit against rent yoiu might owe. These people are taking advantage of you because you are in the service and they figure there's nothing you can do so they'll ream you. Get a tenant's lawyer or go to a tenant's eviction service for free help.