Short answer: if your dogs have fought, keep them apart when you cannot supervise and get help early. A fight can look sudden but often follows signals that are easy to miss.
VCA says dogs often signal very subtly, so early messages may be overlooked until a physical fight. It says fighting between dogs in the same home often starts when the younger dog matures, between one and three years. It also says fear, anxiety and frustration can show up at social maturity.
Keep them apart, with gates or separate rooms, when you cannot watch, because a fight can start without warning. See two dogs fighting at home and growling over dinner.
Watch for the moments that start trouble, such as you coming home or sitting down, and plan for them. The AKC notes that scientists are still debating what jealousy means for dogs, so focus on what you can see.
If your dogs have hurt each other, see your vet, then a qualified behavior professional.
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