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
Due to the fact that Shanks bounty is unknown, and we haven't seen any serious fights that actually went down, we can only estimate Shanks power from his brief appearances, and the dialogues about him:
- He lost his arm to a Sea King in an attempt to save Luffy, then uses Conquerors Kaki(Haoshoku Haki) to scare it off.
- He uses Conquerors Haki to knock out most of Whitebeards crew, then briefly clashes blades with Whitebeard, the strongest man in the world.
- He stops Akainu with his sword and saved Koby.
- When he had two arms, he was in contention with Mihawk as the strongest swordsman in the world. Whitebeard himself said that he still remembers the fights between Mihawk and Shanks, and they echoed all around the Grand Line. However, Mihawk is no longer interested in fighting him.
- In Chapter 907, he went into Mary Geoise to talk with the five elders.
Based on this information, I would say that Shanks is well deserved of his title as Yonko, but not as powerful as the other three. He could also probably take on a Marine Admiral or even the fleet admiral individually. Given that the Navy suffered massive losses already at the battle, I would say that Shanks, along with his crew, is powerful enough of a threat, to stop the war at that point.