The Seven Days’ Battles opens this week 150 years ago in the Civil War.

The weeklong series of battles will consolidate the rise of Confederate Gen. Robert E. Lee and prove influential in shaping the remaining course of the war.

On June 25, 1862, Union Maj. Gen. George B. McClellan sent his combat forces marching toward Richmond, intent on putting the Confederate capital within range of his siege guns.

The Associated Press reported in a June 25 dispatch that the fighting was fierce as Union troops “met with a most determined resistance” in its Confederate foes.

“The ground fought for was a swamp, with thick underbrush,” AP notes.

In such terrain, McClellan’s push is not enough, and Lee goes on the offensive the next day. Lee’s battle plan succeeds in pushing back federal troops, forcing McClellan’s fighters to withdraw southeast along the Chickahominy River.

On June 27, 1862, Union troops clash with Confederate forces at the major Battle of Gaines’ Mill. There, after hours of afternoon fighting, Lee hurls his combined forces in an all-out attack that forces Union rivals to retreat. His is a sweeping tactical victory, his first. But it comes at a great cost in lives. The 15,000 estimated casualties at Gaines’ Mill mark the deadliest and largest battle in the East yet.