Butchery.
Concerning the strategic blunder by Lee at Gettysburg, we see his men pressing the attack to the high water mark... almost breaking the Union line... (again, a singular effort to end the war). In Cold Harbor, we see men driven forth... losing 7000 in twenty minutes... then digging in and having other units push in behind... then they too tried to dig into the open field.
Grant left the engagement lag from 4AM until later that noon. Then, as aforementioned, he left the men in the field to die. Total Union casualties at Cold Harbor almost 13 thousand while the South suffered 5200 (with only 1500 on June 3rd).
Within a month, Lee Army of Northern Virginia was penned up in Petersburg.
almost breaking the Union line... Not even close. a few hundred men from the Confederate forces actually got through the Union Line. The were quickly killed or driven back by a Union counter attack.