I never knew why though. Now I know that April, the first full Spring month, marked the resumption of fighting during wartime.
The second line in “The Waste Land” references death. Lilacs were planted as a memoriam when someone died. Think of Walt Whitman’s poem “When Lilacs Last in the Dooryard Bloom’d” about the death of President Lincoln.
Not surprising that human activity runs counter to what occurs in nature come Spring. Rebirth. Rejuvenation. Death. Destruction.