For decades, the Philippine Air Force hunted communist rebels and Islamist militants in the country’s forests and southern islands. Now, amid seemingly intractable tensions in the South China Sea, it is being reshaped into an armed service meant to defend one of Asia’s most contested maritime frontiers – even as analysts rank it the weakest air arm among Southeast Asia’s six largest militaries. “Because we are an archipelago, we really need to strengthen our air assets,” air force spokeswoman...