When you visit Hakone, the famous hot spring resort near Mt. Fuji, part of the standard tour route is a lake cruise on a gimmicky pirate boat (upper right photo) on Lake Ashi (Ashinoko). As you approach Moto-Hakone Port, you will see this torii in the water at the foot of Mt. Komagatake (upper left photo). This is perhaps Japan’s second-most photographed torii in the water. (The first is Miyajima in Hiroshima.)
It belongs to Hakone Shrine (箱根神社) hidden by trees on the mountain slope behind it. From the pirate boat, the torii looks tiny against the mountain. However, it’s an optical illusion because it’s actually a huge torii.
Get off the pirate boat at Moto-Hakone Port and walk along the waterfront to the torii. You will see that it’s a huge torii (lower left photo). There’s a convenient concrete jetty to see it up close. This torii has an interesting story that I never knew about.
The torii is relatively new, built only in 1952 to commemorate two major events that year: 1. The ceremony that proclaimed Prince Akihito as Crown Prince in November when he became of age. (Akihito is now a retired emperor.) 2. Japan’s independence when the Allied Occupation of Japan ended in April 1952 following the signing of the Treaty of San Francisco (Peace Treaty) in Sept. 1951.
In Oct. 1964, to celebrate the shrine’s 1,200th anniversary and the Tokyo Olympics, the shrine hung a large, framed calligraphy of the word “Heiwa” (Peace 平和) on the top of the torii on the front (lower right photo). The “Heiwa” kanji was fancily brushed by Yoshida Shigeru, Japan’s prime minister who signed the Treaty of San Francisco on behalf of Japan. And so the shrine named the torii, “Peace Torii of Hakone Shrine” (平和の鳥居). There’s also a plaque nearby that says so. Interesting to know that this torii has a Japan-USA connection. 
From Tokyo, the best way to visit Hakone is to buy the Hakone Freepass ticket (valid for 2 or 3 days) to ride everything you need to visit and tour Hakone.
Start from Shinjuku Station and ride the Odakyu Line train (Romance Car) to Hakone-Yumoto. A great tour route awaits. On clear days, you can see Mt. Fuji too. The torii and Hakone Shrine are near Moto-Hakone Port.
Hakone Freepass: https://www.odakyu.jp/english/passes/hakone/
Hakone Shrine: http://hakonejinja.or.jp/