when did he become the president?
September 1901
Theodore Roosevelt
On September 6, 1901, President McKinley was attending the Pan-American Exposition in Buffalo, New York when he was shot by Leon Czolgosz. Roosevelt was vacationing in Vermont, and traveled to Buffalo to visit McKinley in the hospital. It appeared that McKinley would recover, so Roosevelt resumed his vacation in the Adirondacks. When McKinley's condition worsened, Roosevelt again traveled to Buffalo. McKinley died on September 14, and Roosevelt was informed while he was in North Creek; he continued on to Buffalo and was sworn in as the nation's 26th president at the Ansley Wilcox House.

what president said speak softly and carry a big stick Roosevelt which country does he belong to? United States
wiki:19617318
Theodore Roosevelt [SEP] Presidency (1901–1909)
As there was no constitutional provision for filling an intra-term vacancy in that office (prior to ratification of the 25th Amendment in 1967), Roosevelt served his first term without a vice president. McKinley's supporters were nervous about the new president, and Hanna was particularly bitter that the man he had opposed so vigorously at the convention had succeeded McKinley. Roosevelt assured party leaders that he intended to adhere to McKinley's policies, and he retained McKinley's Cabinet. Nonetheless, Roosevelt sought to position himself as the party's undisputed leader, seeking to bolster the role of the president and position himself for the 1904 election.