Napoleon:

In 1808, Napoleon and Czar Alexander met at the Congress of Erfurt to preserve the Russo-French alliance. The leaders had a friendly personal relationship after their first meeting at Tilsit in 1807. By 1811, however, tensions had increased and Alexander was under pressure from the Russian nobility to break off the alliance. A major strain on the relationship between the two nations became the regular violations of the Continental System by the Russians, which led Napoleon to threaten Alexander with serious consequences if he formed an alliance with Britain.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". In what year did the Congress of Erfurt take place?
1808