In 643 AD, Emperor Taizong of Tang was hesitant to cross the sea for a campaign against Koguryo. His general, Xue Rengui, devised a stratagem to get the emperor across and allay his fear of seasickness.
On a clear day, the emperor was invited to meet a wise man. They entered a hall through a dark tunnel and feasted there. After several days of feasting, the emperor heard the sound of waves and realized he had been lured onto a ship! General Xue drew aside the curtains to reveal the ocean and confessed they had already crossed the sea.
Upon discovering this, the emperor decided to carry on and later completed a successful campaign.