How do I fix a team split between veterans and newcomers?

Both camps are protecting something real. The veterans feel their years of hard-won knowledge are being brushed aside, so they dig in. The newcomers feel blocked by walls they did not build, so they push. Neither group is the villain. Each is guarding its own worth, and the friction is what happens when nobody feels safe enough to lower the shield first. You cannot force trust, but you can create reasons for it. The two groups do not actually need to like each other. They need to see that they need each other. The veterans hold the history and the hard lessons. The newcomers hold fresh eyes and new methods. A team that has both is stronger than one with either alone. Put them on something small together where each side clearly holds a piece the other lacks. Pair a veteran and a newcomer on one real task and let them discover the other is useful. Respect grows from working shoulder to shoulder, not from being told to get along.