Firepower of military units large and small has steadily increased since the introduction of firearms, with technical improvements that have, with some exceptions, diminished the effectiveness of fortification. Such improvements made close order formation useless for middle to late 19th century infantry, and the use of machine guns early in the 20th stymied frontal assaults. Military uniforms changed from gaudy to drab, making soldiers less visible to the increasing firepower. At sea, improved naval artillery ended the use of prize crews, and naval aviation brought an end to heavily armored battleships.