Is Shanks that strong? After the War of the Best, when Whitebeard died and there was no hope for the pirate side, Shanks enters the area and everything ends on his command. So I'm wondering if Shanks is really that strong (that three admirals together couldn't defeat him) or do they just honor the saga of Whitebeard?
Shanks is very strong, though that isn't the only reason why the navy stopped. Being one of the four yonko means that his crew is one of the most dangerous in all of the Grand Line. Of course, Whitebeard was at least as strong, if not stronger, but Shanks is no weakling either. He has been able to hold his own against a number of powerful opponents, including at different times and in different situations, Mihawk, Kaido, and Whitebeard. The Marines only came prepared for one Yonko at a time. Facing both the Whitebeard pirates and the Redhair pirates would have been a lot more than they could chew.

Shanks is also not a terribly dangerous pirate in the eyes of the world government, though he is very powerful. They view him as a threat, but acknowledge that he probably wouldn't be the one to start a major conflict on his own. He is respected by a number of people in the navy, including Sengoku, who would rather avoid a conflict with the Red-hair pirates. The navy probably would have been able to win against the Red-hair pirates at that point, but they would have sustained heavy losses in the process which were simply not worth it. He is also rather charismatic, so that probably helped.

Ultimately, Sengoku decided to end the battle out of respect for Shanks, not necessarily because they could not have beaten him, but he is definitely still one of the most powerful people in the world.

Source: One Piece Wiki