Which world war ii event occurred first?
the united states dropped two atomic bombs on japan.
president harry truman was sworn in to office.

President Harry Truman was sworn into office.

Explanation:

Harry Truman was sworn into office as the 33rd U.S. president on April 12th in 1945, following the death of President Franklin Roosevelt. While the two atomic bombs were dropped on Hiroshima and Nagasaki three months later, on August 6 and August 9, respectively, of that same year. Inevitably, Japan announced its surrender to the Allies six days later. And the following month, the war finally ended.

2018 means 2018 ce or common era

there have been other eras too including bce, which included way more years.