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The Prussians were reluctant to be drawn fully into the war against Napoleon for a number of reasons, and were firmly engaged in a policy of fence-sitting. From late October to early November 1805 Tsar Alexander I stayed in Potsdam to see if he could push Frederick II towards a more active role in the war.
Prussia was already beginning to move towards this idea after the French violated their treaty by advancing into Prussian territory, and because of increasing international pressure, but the final spur to action was Alexander's visit. Alexander made it clear that he would not leave Prussia until he had extracted a military commitment and an oath of 'immortal friendship' from Frederick.