After the passing of Saul, David was exalted to ruler over the region of Judah and eventually over all of Israel. In the people's view, he had already been the regent over the land anyway. And, they knew that Samuel had anointed him to be the next king (1st Chronicles 11:2). So, he was inaugurated in Hebron, then proved and exalted in Jerusalem. He took the "castle of Zion" and made it his own (1st Chronicles 11:5). It was an event with eternal ramifications. The new king and his new castle were more significant than anyone at that time could possibly have realized.
Now, the existence of a royal court rightly requires a force of mighty men who legitimize and enforce the will of the king. David had a ring of "knights" who were unmatched in their might and feats. Joab conquered and rebuilt Jerusalem. Jashobeam and Abishai both took on 300 men, each one by himself. And, each one... won. Additionally, Eleazar, Benaiah and Uriah were all 3 men of exceptional value as military men. Add to these the almost 50 other super-soldiers listed in this chapter and what you have is a royal roundtable that could accomplish almost any military mission that was presented to them... by God's grace, of course.
There is one particular feat that seems to exemplify both the ability and the commitment of David's best men. 1st Chronicles 11:15-19 tells of a time when 3 of David's captains went behind enemy lines to get water from a particular well in Bethlehem (for David to drink)... just because they knew that he wanted water from that specific source. Of course, David was a man of greater character than to drink that water. He poured it out on the ground as a sacrifice and offering to God... since his men had risked their very lives for it. He knew that he wasn't worthy of such devotion and loyalty.