If you are a residential tenant, you can basically break your lease at any time... you will lose your security deposit, but there isn't a housing court in the country that is going to force you to pay rent once you have moved out and forfieted your security deposit.....
The threat of paying the lease after you leave, while a nice club for landlords to threaten, is basically uneforceable in residential leases.
Depends on the state. Some states are tenant friendly, others are not. If the landlord is able to rent the apartment out quickly, he might not have any real damages so suing you might be a waste of time.