They first became an enemy when they betrayed him after they blew his cover in the Amnesia Custom Story: Abduction. He also found monsters talking to a pile of barrels om the same story. (Additionally, this is the same Custom Story in which Stephano is born− and dropped in a tub of acid for leading him into a Bro.) From then on, they try to discourage PewDie, hide stuff (usually important items), and make sure he doesn't survive the game. There was even once when they collaborated with the Bro to kill PewDiePie. The only time they were ever allies is when PewDie played Lucius, and they plan to kill everyone and, even at one point, they help him in Heavy Rain and The Walking Dead, possibly being a rogue barrel or a spy barrel. Nonetheless, the barrels are still an infamous enemy. Due to this, most bros choose to hate barrels because of PewDiePie's hatred for them, and often nickname PewDiePie's haters as barrels or dismiss spam or rude comments, with "A barrel has breached security".
According to a level made in Happy Wheels, Bros and Barrels lived together in harmony, until the Barrels started to over-populate and began taking over the world.
Often in the games he plays, PewDie blames the barrels in an area for his death or injury, and sometimes he will not notice barrels at first, but will notice them after he was injured or killed. An ear-splitting shriek of 'BARRELS!' will most likely be emitted after this realization. It may not seem like the barrels actually had anything to do with the injury or death, but don't let that fool yo, barrels are evil.
A barrel briefly accompanied PewDie during the final part of his Gary Dark Secrets Chapter 1 playthrough, where PewDie planned to blow it up with explosives, along with some rubble blocking his progress. However, he forgot to put the barrel there, and it survived, so he carried it with him. However, despite coming with PewDie, the barrel displayed it's evil tendencies as usual, saying PewDie would die and that the barrels would take over the world. Sadly it was not brought to justice, as PewDie dropped it to run from a Brute.
The Barrels are ruled by the Emperor Barrel, who PewDie kills in an Amnesia Custom Story.
Some fans speculate that the well known singer Justin Bieber, may be the King of the Barrels. Due to PewDie's subscriber count being more than Justin Bieber's YouTube account, some think that the Bro Army has defeated the King Barrel.
Explosive Barrels actually helped PewDie in his Alice: The Madness Returns walkthrough by blowing up skeleton sharks and some other enemies. In mid to late 2013, he started to see barrels not as enemies, but something that might help him, as said before, and as evidenced in BADASS CLEMENTINE!