Perhaps the most important part of any army, aircraft allow for a wide range of strategies during combat operations. Although aircraft such as hot-air balloons had been used in previous centuries, fixed-wing style aeroplanes came to prominence in the 20th century, with the First and Second World Wars seeing rapid advancements in technology. Biplanes were common but soon obsolete by the late 1930s, where better materials and structural techniques made the monoplane superior. Towards the end of World War II, the Germans made huge strides in jet technology, although too late to make an impact in the war. Since then, technology has continued to rapidly improve and aircraft remain an integral part of armies all over the world.