The Chance of Victory measures the possibility of you being victorious against the current fleet. It will change as you add/modify/remove dragons from your team. Keep in mind that the difficulty of the battle will increase as you defeat more fleets.

 

Chance of Victory is estimated by the power level of your dragons. You can see the power level of your dragon on their icon at the top right corner, accompanied by the symbol of their type. A higher number means a higher power level. Note that dragon rarity also factors in, as dragons of higher rarity generally have higher power levels. If you have a Unique or Rare dragon on a lower level the chances will be better than having a Common dragon on high level.

 

In short, dragons of higher power levels on your team means the Chance of Victory increase. Do keep in mind, however, that the Chance of Victory does not guarantee a win and is simply an estimate. To succeed at Defend Berk, it is advisable to have a variety of Health, Attack and Range Dragons. Test out with different team compositions to see what works best. Good strategy is also a must. For example, use a Target Dragon to attack an Armored Ship, which can only attack on short distance.