Marines would've easily wiped out Red Hair Pirates at that time had Sengoku decided to continue the war. But Marines already achieved their goal (killing Ace and WB), so as a wise leader, Sengoku decided to finish the war to reduce any unnecessary possible casualties addition.
There are literally 5 people as strong as shanks, and the seemingly closest one in terms of strength to Shanks is Beckman only. So it'd be not hard for Marines to erase RHP from existence if they truly insist.
There are literally 5 people as strong as shanks, and the seemingly closest one in terms of strength to Shanks is Beckman only. So it'd be not hard for Marines to erase RHP from existence if they truly insist.