The covenant to deliver possession is enforceable against the landlord when a third party wrongfully remains in possession at the time the tenant is entitled to possession. The landlord’s obligation is to ensure actual possession, not a mere right of possession.
This is the long-established rule in England and is followed by a number of American jurisdictions.
The landlord need only convey the right of possession, not actual possession – unless there is an express lease covenant to the contrary. This rule was developed historically in US jurisdictions but has been largely modified or abandoned, particularly as to residential tenancies.
Many states have adopted the Uniform Residential Landlord-Tenant Act. The Uniform Residential Landlord-Tenant Act in its current form has provisions expressly creating a duty to deliver actual possession.
Although statutes in many states are based on this model act, the model and the various statutes in jurisdictions that adopted it evolve separately and differ more and more over time. The Washington Residential Landlord-Tenant Act does not have these provisions.
 American Law of Landlord and Tenant, Robert S. Schoshinski.