The Grande Armée's order of battle when it crossed the Niemen River was truly pan-European in nature. In addition to the French troops, the Grande Armée had a sizable contingent from the recently annexed Low Countries, the Italian states either run by members of the Bonaparte family such as Naples or Napoleonic creations like the Kingdom of Italy. The latter represented a type of state structure that was nominally independent from France, but in reality client states run by appointees who acted as Napoleon's viceroy. Not only did the Kingdom of Italy fit this mold, so too did the Swiss Confederation and the Illyrian Provinces. There was a small Iberian contingent consisting of troops sent by Napoleon's brother whom Napoleon put on the Spanish throne as well as Portuguese "volunteers" culled from prisoners of war french forces captured in Spain.
"Germans" and "Poles" made up the largest non-French contingents of the Grande Armée. Some of the latter were from the preexisting cohorts from the Duchy of Warsaw, others formed up after the invasion from Lithuanian and Belorussian men dragooned into service. The Rheinbund states, see this map which included the Kingdom of Westphalia run by Napoleon's brother Jerome as well as older states like Bavaria or Württemberg, contributed another large portion of the troops at Napoleon's disposal. Although Russian contemporaries often noted these Poles and Germans were quite notable among Napoleon's troops, it is important to realize that national sentiment was often quite limited or only in an embryonic form. A Bavarian might recognize that a fellow conscript from Berg spoke German, but he might not have much else in common with his fellow Rheinbund ally. The Danes were also Napoleon's ally in Scandinavia, even if they did not contribute much to the actual invasion.
Two of Napoleon's former enemies, Austria and Prussia, also provided a corps each for the Grande Armée. These states were nominally Napoleon's allies in 1812, but Napoleon pretty much forced Vienna and Berlin into the war. The French Emperor did not place much trust in either ally and had the guard non-essential sectors of the front rather than engage on the offensive with the bulk of the army. This bit Napoleon in the end because it meant both the Prussians' and Austrians' forces did not suffer the attrition from the painful summer advance and the retreat, so they each possessed a strong mobilized force that could defect when the timing was right, as the Prussian corps did at Tauroggen in December 1812.
Yorck's defection at Taurrogen underscores a the problematic issue of who actually was Napoleon's ally in June 1812. Many of the leaders of these various states had little choice but to contribute troops to the campaign. Of this multinational army, only the Duchy of Warsaw's troops arguably had a major stake in French victory over Russia. There were rumors among the Duchy's troops that Napoleon was prepared to resurrect the Kingdom of Poland after victory over Alexander I. Notably, Napoleon never really confirmed this postwar plan during the campaign and some of the Duchy's leaders did recognize that Napoleon was non-committal over Poland and maintained covert lines to the Russians after the campaign's fortunes waned. Napoleon's Rheinbund allies likewise were more than receptive to alternative alliances when the disaster of the retreat was apparent, showing the fragility of the French alliance system. The Grande Armée may have been a European army, but the political cohesiveness that allowed for the creation of such a vast force was entirely dependent upon Napoleon's ability to win military victories.
