The statement that best completes the diagram is Salah ad-Din orders an invasion of western Europe.
<h3>What caused the Third Crusade ending in failure?</h3>
The failure was caused because the King Richard's forces were divided on how to defeat Salah ad-Din and forced to return to Europe.
Hence, the statement that best completes the diagram is Salah ad-Din orders an invasion of western Europe.
Therefore, the Option D is correct.
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