When you engage in actual fighting, if victory is long in coming, then men's weapons will grow dull and their ardour will be damped. If you lay siege to a town, you will exhaust your strength.
In 1779, the Spanish and French attempted to seize a British fortress in the Siege of Gibraltar during the American War of Independence. Despite assaulting the city with heavy guns, ships and thousands of troops, the British held firm retaliating with heated shots from cannons, causing mushroom clouds and forcing the enemy to finish the assault. After nearly four years of fighting, the Spanish and French forces retired and Gibraltar was held.