When Lilacs Last in the Dooryard Bloom'd was written about what US president?

It was written by Walt Whitman to mourn the assassination of President Lincoln. Sic transit tyrannis! ; When lilacs last in the dooryard bloom'd, ; And the great star early droop'd in the western sky in the night, ; I mourn'd, and yet shall mourn with ever-returning spring.

; Ever-returning spring, trinity sure to me you bring, ; Lilac blooming perennial and drooping star in the west, ; And thought of him I love.