What does "when in the course of human events" mean?
Thomas Jefferson, the great writer, philosopher, orator, and Founding Father, was largely responsible for drafting the Declaration of Independence in a short period of time, during which the then-nebulous forces that would become the U.S. Continental Army were coalescing around General Washington, and had already fought gallantly against the British in Lexington, Concord, and the bloody Battle of Bunker Hill in Boston. Jefferson, Benjamin Franklin, and John Adams, all great orators and students of history, understood that their decision to take up arms against King George III and the British Empire was very likely to fail. They also understood that the only chance they had of winning this lopsided conflict was to fire up the American colonists, most of whom actually opposed the war and were terrified of provoking the British. The Founding Fathers' task was to motivate their countrymen to fight alongside them.
Just as importantly, Jefferson, Adams, and Franklin realized they could not defeat the British Empire unless they could enlist foreign powers to come to their aid. For that to happen, they needed to cloak their struggle in terms of good versus evil, right versus wrong, and freedom versus tyranny. Jefferson and his colleagues understood that if European powers like France and the Netherlands saw the conflict as a simple colonial rebellion, they would be unlikely to help, because those countries had their own colonies, which they did not want to lose.
When drafting the Declaration of Independence, Jefferson masterfully defined the struggle as one for the ages, and cast the "American rebels" as almost mythical heroes, who were fighting not only for their own freedom, but also for the freedom of all oppressed people, and for a new and more just world order. By using the phrase, "when in the course of human events," Jefferson cast the colonists' struggle as a turning point in human history, and by doing so, he convinced people all over the world that the struggle of the American Colonists was a struggle for all of humanity to rise up from its shackles and throw off tyranny wherever it existed. This is a crucial point, because subjects of monarchies around the world— particularly in Europe—had many of the same grievances against their kings and queens that the American Colonists had against George III.
Jefferson used his heroic language to make The Declaration of Independence more than a political document about a specific struggle. It is a universal call to arms for everyone who wanted freedom, and who felt that their governments did not exist to serve their interests. This proved to be a stroke of genius, because the Declaration of Independence was soon taken up by people around the world who revered the works of political philosophers like John Locke, Montesquieu, and Rousseau. By framing the conflict as a struggle for progress, Jefferson and his colleagues succeeded in making the British look like bullies and making the colonists seem like the rightful heirs of destiny.
This propaganda coup helped convince wealthy individuals and sovereign states like France and the Netherlands to give much-needed money and military aid to the American Revolutionaries. Without this help, the colonists almost certainly would have lost the war and been branded as traitors.
The phrase that you are asking about “when in the course of human events,” is the beginning of the Declaration of Independence. The phrase means something like “when, as human history unfolds…”
The Declaration of Independence was meant in part to explain to the world why the American colonies wanted to break away from Great Britain and become independent. The leaders of the American Revolution felt that they needed to convince the world that their cause was just and that it was right for them to break away. If they could do this, they might be able to get help and support from other countries. Because the American leaders wanted to explain their actions, they started the Declaration by saying that when a people want to break away from their country and become independent, they need to explain why they are doing so. This is where “when in the course of human events” comes in.
One definition of “course” in the dictionary is “the way in which something progresses or develops.” Using that definition, we can restate your phrase as “when, in the way that human events develop” or, as stated above “when, as human history unfolds.”