Mattis' military career in the Marines spanned 4 decades. He fought in the Gulf War, the Afghanistan War and the second Iraq War.
His final job in the military was as Commander of US Central Command, which oversaw operations in the Middle East. He retired in 2013.
Noboru Yamaguchi, a retired Japan Ground Self-Defense Force Lieutenant General, has met Mattis. He's an expert of national security and Japan-US relations.
Yamaguchi says Mattis was nicknamed "Mad Dog" because he aggressively tackles his mission, and it's not an insult.
"Based on my background as a member of the JSDF, I would say that James Mattis is an excellent military man. There's no doubt that Mattis has the qualities of a fine leader," he says.
He added that Mattis is an intellectual man. Mattis is known for moving his 7000-volume library around with him. He's been referred to as a "Warrior Monk" because he's focused his life on military study, and has never married.
A month ago, Mattis discussed threats to the US and world order.
"I think it's under the biggest attack since World War 2 sir, and that is from Russia, from terrorist groups and with what China is doing in the South China Sea."
He continued, "We must also embrace our international alliances and security partnerships. History is clear: nations with strong allies thrive and those without them wither."
Yamaguchi says Mattis chose Korea and Japan for his first overseas trip as US Defense Secretary to make a statement about the region.
"Mattis' visit shows that the US still values its allies, and that Japan and South Korea can look to the US for support. It's a wise move, especially given the current threat from North Korea, and the difficulty of predicting how China will behave," he says.
Yamaguchi expects the visit will be a good chance to reconfirm the importance of the Japan-US alliance.
"The alliance between Japan and the US isn't just important for the security of the Korean Peninsula -- it's significant for the stability of Asia as a whole," he says.
"That's why the relationship between the US and Japan has been described as the cornerstone of Asian stability. I hope Mattis' visit will cement this idea, and help us to feel confident in our relationship with the US," he continues.
After Mattis' visit, it will be the new president's turn to meet with a leader from the region.
Prime Minister Abe will visit Washington and hold a summit with Trump next Friday.