The exchange is between Pope Pius XII and Emperor Showa, whose given name was Hirohito. They were written after the San Francisco Peace Treaty brought an end to the U.S. occupation of Japan.
In reply, the emperor expresses his deep gratitude on behalf of the Imperial Family and the people of Japan.
返信として、(昭和)天皇は自身の深い感謝の意を表しています、皇室および日本国民を代表して。
The emperor is said to have sought relations with the Vatican two months before the attack on Pearl Harbor in 1941. They established ties the following year.