"Lest we forget" is a phrase commonly used in war remembrance services and commemorative occasions in English speaking countries. Before the term was used in reference to soldiers and war, it was first used in an 1897 Christian poem written by Rudyard Kipling called "Recessional."
RECESSIONAL
by Rudyard Kipling
God our fathers, known of old,
Lord of our far-flung battle-line,
Beneath whose awful Hand we hold
Dominion over palm and pine—
Lord God of Hosts, be with us yet,
Lest we forget—lest we forget!
[first stanza of a five-stanza poem]
Why do we say, “Let we forget”?
As Remembrance Day arrives [now known as Veterans Day], we often hear the commemorative phrase "Lest we forget" as we remember fallen soldiers from World War I across the British Commonwealth. The phrase encapsulates our desire to remember the past tragedy and sacrifice and ensure that such bloody catastrophe never happens again. Nov 11, 2018
In other words, September 11, 2001, should not be forgotten,
that we should remain vigilant, that it might never happen again.