After losing the election to Woodrow Wilson, Roosevelt and his son Kermit embarked on a voyage of discovery in the jungles of Brazil, where he almost lost his life. During this time, he also wrote scientific essays and more history books, and led the cause for military preparedness, convinced that the nation would have to go to war once World War I broke out in Europe. He was greatly disappointed when his offer to lead a regiment was turned down by the war Department. When his young son, Kermit, an aviator pilot, was shot down and killed while flying a mission in Germany, it is said that Roosevelt finally lost his boyhood. Thereafter, although he continued to stump the nation making speeches in favor of war bonds and the war, his mood and voice were less enthusiastic than before. for the first time in his life, sadness overtook the once unconquerable warrior.

Theodore Roosevelt died in his sleep on January 6, 1919, at home, in his beloved house at Sagamore Hill in Oyster Bay, New York. One commentator said that death had to take him while he slept else it would have had a fight on his hands.

He was only sixty-one years old when he died, and the "strenuous life" had taken its toll. Always pushing his life to the maximum, Roosevelt had suffered a detached retina while boxing (as president) that blinded him in one eye. He carried a bullet in his chest from an assassin's attempt on his life while making a speech in the presidential campaign of 1912. He had contacted malaria and suffered a serious leg infection while exploring Brazil on his seventh- month, 15,000 mile expedition in 1913. And his love for swimming in the freezing waters of Oyster Bay and the Potomac River during winter months left him with painful inflammatory rheumatism and an ear infection that ruined his hearing in one ear. The coronary embolism that claimed his life took him early in the morning, just as the morning haze that he so loved to watch rose off the ocean's waters at Oyster Bay.