Destroyer doctrine starts with the mission

A navy's missions establish what its destroyers must do. Capital ships, carriers, cruisers, and coastal-defense ships impose different escort and cooperation requirements. Dependence on maritime routes also matters where sea transport connects islands or substitutes for roads and railways. Treaty obligations to protectorates, dominions, and other territories add commitments that the fleet must support. Budgets and available ships constrain the doctrine a navy can actually carry out. An intended fleet that has yet to be built supports plans and ambitions, rather than an immediately usable force.