The simple answer to both questions is World War I. Pax Britannica ended with the First World War, though it wasn't until 1945 that the United States replaced Great Britain as the hegemon. Between those years there wasn't one dominate power, though the US was still better off than other powers. The war hurt all its participants, and after it ended Britain no longer far outmatched the other powers, specifically in terms of naval power which was the backbone of English hegemony. Also, despite how little America actually did to help end WWI, it did break their isolation and bring them into affairs on a global scale rather than just a Western Hemisphere one.