After the congress, the Kingdom of Poland became part of .

Russia

Explanation: After the congress, the kingdom of Poland became part of Russia. Following the partitions of Poland at the end of the 18th century, Poland ceased to exist as an independent state for 123 years. The Kingdom of Poland enjoyed considerable political autonomy as guaranteed by the liberal constitution. However, its rulers, the Russian Emperors, generally disregarded any restrictions on their power. It was, therefore, little more than a puppet state of the Russian Empire.