Russia, though nominally an ally in the Fourth Coalition, remains a dormant entity for much of 1806 (giving virtually no military aid to Prussia in the main battles that October, as Russian armies were still mobilizing).
Russian forces will not fully come into play in the war until late 1806 when Napoleon enters Poland.
Finally, Prussia had remained at peace with France the previous year, though it had come close to joining the Allies in the Third Coalition.
A French corps led by Marshal Bernadotte had illegally violated the neutrality of Ansbach in Prussian territory on their march to face the Austrians and Russians.
Anger by Prussia at this trespass had been quickly tempered by the results of Austerlitz, and a convention of continued peace with France had been signed at Schönbrunn two weeks after that battle.
This convention had been modified in a formal treaty two months later, with one clause in effect promising to give Hanover to Prussia in exchange for Ansbach's being awarded to France's ally Bavaria.
In addition, on March 15, 1806, Napoleon had elevated his brother-in-law Marshal Joachim Murat to become ruler of the Grand Duchy of Berg and Cleves (acquired from Bavaria in return for its receiving Ansbach).
Murat had exacerbated Prussian enmity by tactlessly ejecting a Prussian garrison that was stationed in his newly acquired realm, prompting a stern rebuke from Napoleon.
Relations between France and Prussia had quickly soured when Prussia eventually discovered that Napoleon had secretly promised to return sovereignty of Hanover back to Britain during his abortive peace negotiations with the British.
This duplicity by the French will be one of the main causes for Prussia declaring war this autumn.
Another cause is Napoleon's formation in July 1806 of the Confederation of the Rhine out of the various German states that constitute the Rhineland and other parts of western Germany.
A virtual satellite of the French Empire with Napoleon as its "Protector", the Confederation is intended to act as a buffer state from any future aggressions from Austria, Russia or Prussia against France (a policy that is an heir of the French revolutionary doctrine of maintaining France's "natural frontiers").
The formation of the Confederation had been the final nail in the coffin of the moribund Holy Roman Empire and subsequently its last Habsburg emperor, Francis II, had changed his title to simply Francis I, Emperor of Austria.
Napoleon has consolidated the various smaller states of the former Holy Roman Empire that had allied with France into larger electorates, duchies and kingdoms to make the governance of non-Prussian and Austrian Germany more efficient.
He has also elevated the electors of the two largest Confederation states, his allies Württemberg and Bavaria, to the status of kings.
The Confederation is above all a military alliance: in return for continued French protection, member states are compelled to supply France with large numbers of their own military personnel (mainly to serve as auxiliaries to the Grande Armée), as well as contribute much of the resources needed to support the French armies still occupying western and southern Germany.
Understandably, Prussia is indignant at this increasing French meddling in the affairs of Germany (without its involvement or even consultation) and views it as a threat.
Napoleon had previously attempted to ameliorate Prussian anxieties by assuring Prussia he was not adverse to its heading a North German Confederation, but his duplicity regarding Hanover has dashed this.
A final spark leading to war is the summary arrest and execution of German nationalist Johann Philipp Palm in August 1806 for publishing a pamphlet that strongly attacks Napoleon and the conduct of his army occupying Germany.
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