The scene where those two Confederate and Union redshirts start commiserating across a river, then walk out to the middle of it and trade a few puffs on a pipe for some coffee. Sure, we're on opposite sides of a war, but what the hell, it's Christmas.
Made all the more heartwarming by the simple fact that, if the two redshirts were caught doing this, they'd most likely be punished by execution for fraternization with the enemy.
Combines with Tear Jerker, but the party at the Hancock household in California, before Winfield leaves to join the Union and his best friend Lewis Armistead leaves to join the Confederates. Mira Hancock, her husband, and their friend have a tearful goodbye, with Armistead asking Mira to play the song "Kathleen Mavourneen" (which he and Winfield both remember fondly).
Another moment that is Heartwarming combined with Tear Jerker, the Irish Brigades from both the Confederate and Union armies clashing at Fredericksburg, at one point in the scene, what appears to be an officer of the CSA army breaks down crying at the sight of dead and dying irish Union soldiers, after he regains his composture, he takes off his hat and starts to yell what looks like an ancient irish word, the rest of the CSA soldiers soon see this and proceed to join the officer in tearfully cheering and praising the bravery of the now retreating Union Irish Brigade.