In the episode "War Without End":
Jeffrey Sinclair, Commander Ivanova, Ambassador Delenn, Marcus and Captain Sheridan travel back in time to Babylon 4.
I did not understand the reasoning explained in a scene explained by Jeffrey Sinclair to Garibaldi at all.
In the scene, Jeffrey Sinclair left Garibaldi a private voice message, stating he did not tell Garibaldi he was on Babylon 5 because Jeffrey believed that Garibaldi would either try to stop him or join him on the mission.
Commander Ivanova was allowed to go on the mission and she was equally as important in the timeline as Garibaldi was.
What was the reasoning behind Jeffrey Sinclair not wanting Mike Garibaldi to not join them on the War Without End mission?