Wars encompassing more than one area of the globe aren't exactly new, one can look back as far as the Seven Years War to see a conflict which took place across multiple continents. The seven years war involved fighting on the European continent, North America, and the Indian subcontinent. Another war of great magnitude was the Crimean war in the 19th century, which involved the French, British, Russian and Ottoman empires. Wars became global once those belligerents able to engage fellow great powers had influence across the globe, and while the scale of the world wars far eclipsed previous conflicts, wars which took place over massive areas with multiple belligerents have occurred since the early modern era, the ability to do so can be traced back to when governments acquired the ability to project influence beyond their own homeland, which I would posit began with the Spanish and Portuguese colonization of America in the 16th century.