America at 250 remains the world's pre-eminent power, but growing doubts over its leadership, alliances and credibility have raised questions about whether it can sustain the global order it created.The United States turns 250 as the most powerful nation on earth. The story of its rise was written in treaties, dollars and aircraft carriers. The story of its perceived decline is being written in tariffs and the “quiet quitting” of old friends.US@250: Primus inter paresTo paraphrase Abraham Lincoln at Gettysburg: Twelve score and ten years ago, the founders of an upstart republic brought forth a new nation conceived in Life, Liberty and the pursuit of Happiness. They could not have imagined that it would go on to remake the modern world in its own image. But for how long? That is the questio