What does Lincoln say that is ironic in the Gettysburg address?

In the Gettysburg Address, Lincoln notes that the Civil War is testing whether a nation "conceived in Liberty, and dedicated to the proposition that all men are created equal" can endure. The irony lies in the fact that the very nation founded on these principles was simultaneously engaged in a brutal conflict over slavery and inequality. Furthermore, he emphasizes the sacrifice of the soldiers who fought for this ideal, suggesting that their deaths would not be in vain, yet highlights the ongoing struggle to truly achieve the equality and freedom the nation professes to uphold.