Were there any battles in history which saw the tide turned due to unexpected intervention of allies who appeared to repay a debt of gratitude?
In Chinese history, circa 646 BC, Lord Hui of Jin, disregarding the famine food aid received from neighbouring State of Qin the previous year, not only refused to repay the kindness when it was Qin's turn to suffer the drought, but took the opportunity to launch an invasion.
In the ensuing battle, Lord Mu of Qin was almost captured by the Jin army when "mountain barbarians" suddenly appeared and attacked the Jin forces.
Taken by surprise, the Jin forces were beaten back and the tide of the battle turned. In a separate engagement, the Lord of Jin was captured by Qin forces.
It turned out that the "mountain barbarians" were repaying a debt of kindness to Lord Mu of Qin.
Several years before, they had stolen some horses belonging to Lord Mu and slaughtered the steeds for a feast.
Rather than launching a punitive team against them, Lord Mu sent them some wines, informing the "mountain barbarians" that the brew would moderate the ill-effects of eating the horse flesh.
When the "mountain barbarians" learnt of Lord Mu's war, they took the initiative to lend him their aid. After the war was over, they refused all forms of rewards and returned to the mountains.
Are there any accounts such as these from other parts of the world?