How can you explain the gap between Napoleon and Alexander after Tilsit?

Their gap can be explained by the fact that contradictions arose between them. So Tsar Alexander broke the blockade of England, forbade his niece to marry Napoleon. There were also other disputes that led to the outbreak of the war. Thus, in 1812, Napoleon’s army marched against the Russian Empire. Despite the first major victories, nevertheless, Napoleon’s army was not designed for a long military campaign, and therefore, a significant part of the troops deserted or died from diseases.