A successful Gallipoli removes the crisis that led Churchill to resign and serve on the Western Front. Remaining in government could put him in contention with Lloyd George if Asquith fell in December 1916. That is one branch of the counterfactual rather than an inevitable succession. Military success might also strengthen Asquith enough to remain in office, leaving Churchill's later path different again.
Notable Point · 1:55:58-2:00:49
How a successful Gallipoli could change Churchill's political career
Success removes Churchill's resignation and could make him Lloyd George's rival if Asquith's government still fell.