Complex sentence: Haig opposed the plan because of doubts about inexperienced New Army divisions assembling on the battlefield at night but eventually deferred to Rawlinson and the corps commanders, after modifications to their plan.
Split into simpler sentences: Haig opposed the plan because of doubts about inexperienced New Army divisions assembling on the battlefield at night. Haig eventually deferred to Rawlinson and the corps commanders after modifications to their plan.