A lesson as old as Thucydides is that there is no defensive weapon that is not effectively also offensive. If you have a long wall connecting your city to your harbor, that means that you need not fear attack and will feel free to become an absolute nuisance. Also you need fewer soldiers to defend yourself, so this frees up soldiers for operations elsewhere.

'Ukraine' can defend 'Ukrainian' cities (many of which were founded by Russians) by stopping its warmongering.