This post follows on from this one on the military aspects of the actual invasion.
Russia and France were allied in 1807 by the Treaties of Tilsit, which also involved Prussia. Tsar Alexander I of Russia agreed to comply with the Continental System, Napoleon’s attempt to wage economic war on Britain. Alexander supported Napoleon when he went to war with Austria in 1809, sending an army to threaten Austria’s eastern frontier. His foreign minister, Nikolay Rumiantsev, thought that the alliance was in Russia’s interests.
Charles Esdaile argues that Napoleon damaged Franco-Russian relations by making too many demands of his ally. He wanted Russia to send troops to the West, and away from Serbia and the Danube, where Russia was fighting the latest in a long series of wars with the Ottoman Empire. Napoleon ignored Russia’s interests and left Alexander feeling that he was regarded as the junior partner in the Alliance.
The Poles under Marshal Joseph Poniatowski, unlike the Russians, gave great support to Napoleon in 1809. Poland had been first weakened and then destroyed after its territories were partitioned by Austria, Prussia and Russia in 1772, 1793 and 1796. The Treaty of Tilsit between France and Prussia in 1807 had established the Grand Duchy of Warsaw as a French satellite; it consisted of most of the former parts of Poland annexed by Prussia. In 1809 it was expanded by the addition of territory taken over by Austria.
Alexander, wanting to retain the lands that Russia had acquired when Poland was partitioned, saw the growth of the Duchy of Warsaw as a threat. It was not called Poland but used Polish symbols and the Polish language. He tried to negotiate a treaty under which Austria, France and Russia guaranteed the Duchy’s current borders and agreed that it could not call itself a kingdom. Napoleon refused to sign on the grounds that this meant that the actions of another state could force France into a war. Esdaile points out that, whilst this was true, Napoleon’s real motivation was to allow himself freedom of action in Eastern Europe.
Many Russian were also concerned by King Charles XIII of Sweden’s adoption of Jean-Baptiste Bernadotte, one of Napoleon’s marshals, as his son and heir in May 1810. The Russo-Swedish War of 1808-9 resulted in Sweden losing Finland to Russia. King Gustavus IV, who was considered to be insane, was deposed and replaced by Charles, who was childless and in poor health. Sweden still controlled over half the Baltic coast, including Pomerania, and many Russians feared that Napoleon was trying to encircle Russia.
In fact, although Bernadotte had served with Napoleon for many years and he and Joseph Bonaparte were married to sisters, he was not close to the Emperor. Bernadotte was jealous of Napoleon’s success, whilst the Emperor had been angered by Bernadotte’s failure to move his corps to the action at the Battle of Jena-Auerstadt and by his attempt to claim credit for the victory at Wagram. According to Zamoyski, Napoleon once said that he would have had Bernadotte shot three times had they not been related by marriage.
A further source of dispute was Napoleon’s desire for an heir. He and the Empress Josephine had failed to have children. Since she had a son and a daughter by her previous marriage, it appeared that this was the fault of the Emperor until he had a son by Maria Walewska, his mistress. Determined to produce a legitimate heir, he decided to divorce the 46-year-old Josephine and marry a younger and royal woman.
Napoleon’s first choice was Alexander’s teenage sister, Grand Duchess Anna. Their mother, the Dowager Empress, opposed the intended marriage, as did many leading nobles, who did not want to strengthen ties with France. Alexander was not keen and in February 1810 asked for a two-year postponement because Anna was too young. Napoleon immediately turned his attentions to the Archduchess Marie-Louise, daughter of the Austrian Emperor France I, marrying her on 1 April 1812.
Napoleon managed to offend both the Russians by his quick change of target, and the Austrians by the speed and lack of courtesy with which he pursued Marie-Louise. According to both Esdaile and Zamoyski, he was not playing a double game; Anna was his first choice, but he wanted to marry a young princess and father an heir as soon as possible. There were some complaints in France about Napoleon breaking his ties to the Revolution by marrying an Austrian archduchess, but this had little political impact.
One strong reason for Russian anger with France was the impact of the Continental System. Russia had little industry and had to import many manufactured goods. Alexander wanted to expand Russian trade, but exports fell by 40 per cent in 1806-12, customs revenue from 9 million roubles in 1805 to under 3 million in 1808, the paper currency halved in value in 1808-11 and prices of coffee and sugar rose by as much as five times in 1802-11.
Alexander did not leave the Continental System, but he ceased to enforce it. In December 1810 he opened Russian ports to US ships and made tariffs on imports by land, such as French goods, much higher than those on goods, mostly British, coming by sea.
Also in December 1810, Napoleon annexed the free ports of Hamburg, Bremen and Lübeck in order to give him greater control over imports into Europe. The next month he took over the Duchy of Oldenburg. Its ruler, Grand Duke Peter, was Alexander’s uncle, and Peter’s heir was married to one of Alexander’s sisters, Grand Duchess Catherine.
By 1811, Alexander was preparing for war with France. He offered to restore the Kingdom of Poland, but the Poles did not want a war that would take place mainly on their territory, and many of them preferred Napoleon to Alexander. Attempts to sign military alliances with Austria, Prussia and Sweden failed. Alexander decided that Russia would not start a war with France.
It was Napoleon who decided to go to war. The best explanation for the reason why is that given by Charles Esdaile:
‘One is left, then, with one explanation, and one explanation alone: frustrated by the long war in Spain and Portugal, and the failure of the Continental Blockade to bring the British to heel, Napoleon was simply bent on flexing his military muscle and winning fresh glory’
 C. J. Esdaile, Napoleon’s Wars: An International History, 1803-1815 (London: Allen Lane, 2007), pp. 401-4.
 Ibid., pp. 406-7.
 A. Zamoyski, 1812: Napoleon’s Fatal March on Moscow (London: HarperCollins, 2004), p. 66.
 Esdaile, Napoleon’s Wars, pp. 408-10; Zamoyski, 1812, pp. 56-57.
 Esdaile, Napoleon’s Wars, p. 414.
 Ibid., p. 458.