A few months ago our dog had a small litter of puppies .. one girl and 2 boys. The girl went to her new home with a family friend at 12 weeks of age. We decided to keep the two boys. They’re now 5 months old and seem to have suddenly turned on each other. They used to get on really well and would play a lot and would be fine. Then on the odd occasion a play fight would turn nasty and they’d start fighting for real, but as soon as we separated them it was forgotten about and they were fine again. But for the last 2 weeks they haven’t been able to stay in the same room as each other without fighting. And I’m not talking play fighting or just barking at each other, I mean full on vicious dog fighting every time they see each other. We’ve tried separating them for a few days but then as soon as we reintroduce them they go for each other as soon as they make eye contact. So then we changed tact and tried keeping them in the same room so that they could get used to being around each other but both tied on leads so they couldn’t reach each other... this didn’t seem to do much good either. They’d have periods where they’d sleep or lie down quietly, but as soon as they made eye contact again they’d start snarling, aggressive growling and trying to get at each other to fight. We’ve done positive reinforcement... a treat, lots of praise and love and cuddles when they’re good and don’t snarl each other but it’s rare that this happens, they start snarling and growling the second they lay eyes on each other. We can’t work out which one is the aggressor as both of them have instigated the fighting on different occasions, it usually just takes one to lock eyes on the other and start growling and they’re off. We also can’t figure out what’s triggering the fighting. They get fed in separate rooms so we don’t think it’s food. Has anyone else experienced this? Is it a common thing? We’re considering getting one or both of them neutered, but I’ve read a few different articles saying that this doesn’t always work. We adore them both but if this carries on then one of them will have to go to a new home as it’s impossible to live like this, and if we left them to it I genuinely think one of them would end up dead. We’re desperate to sort this so that they can both stay, is this a realistic expectation? Any advice or ideas greatly appreciated Forgot to mention the breed.. they are border collies and are 20 weeks exactly.