It's a surprisingly tricky question. Governments change their constitutions, get conquered, shift from monarchies to republics, gain or lose territory, and evolve over time. So historians look at a few different measures: the institution (has it ever been abolished?), the constitution (has the founding document ever been replaced?), and sovereignty (has the country ever lost independence?).

Those measures rarely point at the same answer. The Papacy wins on institutional continuity but has changed from a territorial power ruling much of Italy to a city-state of a few hundred residents. Japan's monarchy is the longest by dynasty, yet the emperor held little real power for most of that span. San Marino has the oldest continuous republic, and the United States the oldest written constitution still in force, two very different claims.

Dates themselves are contested. Founding years like Japan's 660 BC or San Marino's 301 AD come from tradition rather than records, and historians often accept a later, documented date instead. The bar chart below uses conservative figures, and even those are approximations.

The governments profiled here are the strongest contenders under each measure, each with an honest note on what the claim means and where the caveats lie. None of them has run unchanged; the point is that the thread was never fully cut.