By crossing the Rubicon River, he knew he would be effectively declaring war on the Roman Republic. This was going to begin a Civil War and pit the country against itself. At this moment, before crossing the river, Caesar is reported to have quoted a Greek play and said The die is cast. By this, he meant that crossing this river was the point of no return because when dice are created, there is no way to change them.
It relates to Julius Caeser crossing the Rubicon River with his army. A big no no at the time.
See more here.
https://study.com/learn/lesson/julius-caesar-crossing-the-rubicon.html
By crossing the Rubicon River, he knew he would be effectively declaring war on the Roman Republic. This was going to begin a Civil War and pit the country against itself. At this moment, before crossing the river, Caesar is reported to have quoted a Greek play and said The die is cast. By this, he meant that crossing this river was the point of no return because when dice are created, there is no way to change them.
And here, https://en.m.wikipedia.org/wiki/Crossing_the_Rubicon