For a quarter century, the U.S. and its allies owned the skies, fighting wars secure in the knowledge that no opponent could compete in the air. As tensions with Russia and China surge, that's no longer the case.

Rapid technological progress in China's aerospace industry, particularly air-to-air missile systems fired from an aircraft, is changing the game for Western air forces and the global arms trade. It's also altering the picture for China's neighbors such as India.

Russia took the lead in modernizing its air force, and has been more willing to use it. In the longer term, however, China's roughly $13 trillion economy and growing wealth mean it is likely to pose the greater strategic challenge for the U.S. and its allies. In 2017, Chinese defense spending rose by 5.6 percent in constant U.S. dollar terms, while Russia's fell by 20 percent, according to the Stockholm International Peace Research Institute. China spent $228 billion last year and Russia $66.3 billion, SIPRI said.