What country has the longest border with France?

The answer is Spain. Regardless of What form of goverment either france or Spainish had either Monarcy or republic these two states have shared this border since Spain was constituted in 1492 after they Conquered the Granada state from the Muslims. The Belgium nation is relatively brand new by this comparison having been constituted in 1830. before this Austrians and Spanish people also had border France in what is now Belgium. also the Dutch from 1815 to 1830 occupied this border. Theres also some Minor city states along the borders as well as unreconized states. Andorra and monoco have shares a border with france for centuries. there was a Un-recognized state called Goust which was near Frances border with Spain. Goust Compares to Transnistria or even Kosovo today in the idea that it was not Fully reconized as a country. Italy is a relativly brand new Country Prior to Italy annexing rome city in 1870 the country of Italy was divided into a number of Independent sovereign provinces. if we also include non-land borders the true answer to the question would be the state of England has had the longest existing border with France. But it would be England itself and not the United Kingdom. The united kingdom has only been around since 1707.

Brazil and French Guiana have shared a common border for centuries as well. except for a breif instance in the year 1814 where Portugal on behalf of Brazil invaded and occupied French Guiana in 1814.