Jahn had hoped to get a footing in the university at Göttingen, it seems; but in the fall of 1806, while he was sojourning in the Harz Mountains, his plans were suddenly changed and his thoughts turned from the classroom to the camp. Napoleon’s incursions and his insolent treatment, which were threatening the very existence of Prussia, had at last compelled Frederick-William III to take up arms against the French. Learning that war was inevitable, Jahn immediately gave up his visit and hastened toward the army which was gathering in Thuringia, intending to volunteer his services. Delayed by swollen streams, he reached Jena on the day of the battle (October 16, 1806), saw the last struggle and the crushing defeat, and joined the fleeing soldiery as a “volunteer fugitive.” Frederick-William’s ill-prepared and poorly officered troops had proved an easy prey for the French. One by one the great fortresses were surrendered and garrisoned by the enemy’s forces, the royal family was forced to flee from Berlin, and by the Treaty of Tilsit (July, 1807) Prussia lost half her territory and became virtually a mere province of France, overrun by the soldiers of the conqueror and entirely at his mercy. Jahn’s flight and later wanderings led him by wide detours to Halle and Magdeburg, down the Elbe, across to the Baltic and westward along the coast to Lübeck (November 5), thence into Silesia, apparently, and back again to Jena, where he remained until the Treaty of Tilsit. Afterwards, until the fall of 1809, he spent a portion of his time at home, but lived for the most part with a friend at Dammeretz, on the Elbe some distance below Lanz. Before the outbreak of the war he had been engaged on two works, a “Handbook for Germans,” and “German Nationality” (_Deutsches Volksthum_); but the manuscript of both seems to have been lost in the days following the battle of Jena. The former was not rewritten, but the latter, his chief literary work, was again ready in 1808 (the introduction is dated October 14, at Lanz) and appeared at Lübeck in the spring of 1810. Its central thought is the unity of Germany, and in its pages his controlling passion for the German language, customs, and history, his intense love for the fatherland, and his desire to see it bound together into one strong nation, able to throw off the foreign yoke, found full and forcible expression.