A couple details from Wikipedia:

“Ue o Muite Arukō” was written by lyricist Rokusuke Ei and composer Hachidai Nakamura. The lyrics tell the story of a man who looks up and whistles while he is walking so that his tears will not fall, with the verses describing his memories and feelings. Ei wrote the lyrics while walking home from participating in the 1960 Anpo protests against the U.S.–Japan Security Treaty, expressing his frustration and dejection at the failed efforts to stop the treaty. However, the lyrics were purposely generic so that they might refer to any lost love….

The song spent three weeks at the top of the American Billboard charts in June 1963. It has been described as a metaphor for the emerging post-World War II global expansion of Japan onto the world scene.