I want to test the ability of students to read a passage and answer questions about it. Could you please come up with a good question for the passage "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."?
Answer:
what event is first mentioned?