Twentieth century witnessed the rise and fall of many great powers. With the start of that century, nations fought and won battles individually to stretch their empires and economics. Within the same century, when the sizes of a few empires rose to mythical proportion, others vanished from the map. It was during the same twentieth century, that the alliance building becomes a religion. Both World wars and the entire Cold war period were dominated by the principle of 'building and maintaining alliances'. Two countries, the United States and the United Kingdom, understood the art of alliance building more than any other nation. From the last decade of the twentieth century till Iraq war, the world witnessed a series of events, one after another. Cold War ended, Russia disintegrated, US-led alliance won Gulf war, NATO's existence, expansion and future (if any) were clouded with questions marks' Ireland issue erupted; Kosovo, Bosnia-Herzegovina and Yugoslavia crises deepened; Euro currency started and September 11 and subsequent Afghanistan and Iraq Wars occurred. It was during these eventful years, the US and UK stood together to shape the course of history. Their mutual alliance essential for each other but indispensable and failed to achieve their objectives. The book "Essential Alliance" chronicles the success and failures.