Richard III movie and play

While i watched and read the ending scene of Richard III, the line A horse! a horse! my kingdom for a horse! stood out to me. What was the true allusion behind this line i thought to myself, being a Shakespeare play i was certain that i had a deeper meaning. After doing some web searches and asking around, i realized that the line probably means what I originally thought it meant. Richard's horse had been injured so he was unable to progress, this meant that his life was now uncertain and the kingdom he worked so hard to gain would be stripped from him.