Term used by political scientists and historians
The term foremost power has been used by political scientists and historians to describe the allegedly greatest power in the world, or in a given region, during a certain period of history. Multiple empires have been described as such, often for the same time period, resulting in a problematic assessment of the conflicting scholarly opinions and points of view on the matter. There is therefore a general lack of consensus between the various authors and scholars in reference to the nations and empires that were allegedly the world's most powerful at various points in history.
The status of foremost power globally implies that of superpower. However, it does not necessarily mean that the world is unipolar nor that there is a sole superpower. Currently, the United States is no longer an uncontested superpower, partly due to not dominating in every single domain (i.e. military, culture, economy, technology, diplomatic) in every part of the world. Although it is still the most powerful military and has the largest economy by nominal GDP (although China has surpassed the United States in GDP purchasing power parity, and could surpass the United States nominal GDP in the coming decades), China has made significant gains in cultural influence and technology. The United States became the world's foremost power at the end of the Second World War, as the Soviet Union was a power of comparable influence, but lagged far behind the United States in economy and wealth. The United States remained the world's foremost power until the dissolution of the Soviet Union in 1991, at which point it became the world's sole superpower. Opinions differ on when China's rise changed the United States' position from an uncontested sole superpower to a contested one. However, most agree that this happened sometime in the late 2000s or early 2010s post-Great Recession. While China's rise decreases the power gap between them and the United States, the United States is forecasted to remain the world's foremost power for the next couple of decades.
According to the Asia Power Index 2023, the United States still takes the lead on the "military capacity", "cultural influence, resilience," "defense networks, economic resources, and future resources" but lags behind China in the two parameters of "economic relationships" and "diplomatic influence" across eight measures in Asia. However, the United States remains ahead of China in each of these categories on a global scale. The term "potential superpowers" describes polities that could rival American primacy in the future.
History.
The label of foremost power has been given to different empires that even co-existed at their peak. This can be explained by the fact that different scholars work on different sources and have different perspectives on what makes an empire the world's most powerful. For the first century, the term has been applied to the Roman Empire, which governed much of the ancient world for over five centuries, giving the city of Rome the nickname of Caput Mundi (Head of the world). 
Giovanni Botero, one of the first scholars of international relations, identified the Papacy, head of the Roman Catholic church, as the foremost power of the Renaissance and Counter-Reformation era. 
In reference to the 16th and 17th centuries, when globalization emerged, the term has been applied to a variety of states including: the empires of Portugal, Spain, and their Iberian Union, and later that of the Netherlands, which all ventured out from Europe to establish colonies in the Americas, Africa and Asia; the Austrian Habsburgs in Europe; the Ottomans, Mughals and Safavids in the Middle East and Central Asia; and the Ming dynasty in the Far East.
During the 19th and early 20th centuries, Britain emerged as a foremost power, driven by its vast colonial empire and industrial prowess. Its naval supremacy, exemplified by the Royal Navy, ensured control over crucial trade routes and maritime dominance.
During the Cold War, the term has been applied to both the United States and the Soviet Union. The concept of superpower, developed in order to describe these two nations and (earlier) the British Empire, became more common than that of foremost power largely because it was not possible to identity a single hegemonic force in global affairs. This can be considered true for most of history as well.
References.
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