Part of the fun in wargaming is fielding a force to complete an objective against whatever force your opponent devises to stop you. Whether you are planning a pitched battle, a what-if scenario, or a historical recreation, you’ll need to execute a strategy if you want to win.
Sometimes, your strategy will be dictated by the types of troops you put in your army list, or by tactics employed during historical conflicts. You might, however, try to begin with a strategy, use that to inspire your army list, and then try to carry out that strategy with troops chosen for the purpose.
One resource for choosing a strategy is the book How Wars Are Won by military historian Bevin Alexander. In that volume, the author cites several military axioms that could inspire a tabletop wargamer when selecting troops.