The foursome that came together and created Killswitch Engage already had strong fan followings. Mike D'Antonio (bass) was formerly the leader and principal songwriter of Overcast, the legendary underground metal group. When Overcast split in 1998, D'Antonio sought for a year to find the right combination of players to fuse hardcore and metal with melody. During the summer of 1999, D'Antonio connected with Adam Dutkiewitz, who was the drummer for Aftershock, and Joel Stroetzel, Aftershock's guitar player. Jesse Leach, vocalist for Corrin and Nothing Stays Gold, joined on, and the foursome took the name Killswitch Engage.

During 2001, the band recorded "Numbered Days" for WERS' Nasty Habits live CD, a compilation with bands such as God Forbid, Haste, Unearth, and Poison the Well. Also in 2001, Killswitch Engage signed with Roadrunner Records and started recording their second full album, Alive or Just Breathing, at the Zing Studio in Westfield, MA. They expanded to a five-piece with the addition of former Aftershock drummer Tom Gomes, as Dutkiewitz moved over to guitar. The album included favorites such as "Self-Revolution," "Just Barely Breathing," and a re-recorded version of "Temple From the Within." Andy Sneap was signed on to mix and master this one in his English studio. The album aimed at even more of the growling bass tone, heavier guitar crunch, and thicker/faster double bass that permeate the music of the band, and it was greeted with high praise upon its release in May 2002 (it even shot to number 37 on Billboard's Heatseeker Chart).

Tours with Soilwork and Hypocrisy finished before the band suffered a setback that June; lead singer Leach suddenly left the group, citing voice problems and personal issues. (He later went on to join metal outfit Seemless.) Breaking up was not an option, but Killswitch was still unsure of their future -- until auditioning Howard Jones that is, vocalist for local metalcore outfit Blood Has Been Shed. The band and Jones clicked immediately, and his charismatic yet menacing stage presence sealed the deal. He made his debut with the band at summer 2002's Hellfest, and the guys stayed on the road for the remainder of the year, both home and abroad, winning over skeptics along the way. Gomes exited the group following their stint on 2003's Ozzfest and he was replaced by another Blood Has Been Shed alum, drummer Justin Foley.

The End of Heartache appeared in spring 2004, debuting at number 21 on the Top 200 and even earning Killswitch a Grammy nomination for Best Metal Performance. Much touring followed, including acting as main support for Slayer and spots on package tours like Ozzfest and Taste of Chaos, while the band also released the DVD Set This World Ablaze in late 2005. Killswitch Engage further rocked a main stage spot at the U.K.'s Reading festival in summer 2006; all of this activity ultimately led up to the release of their fourth full-length, As Daylight Dies, that November.