The Angelic Doctor* points to the diversity of objects that are united within a single order as being evidence for the existence of God. There are obvious examples of this: gravity, electromagnetism, and the strong and weak atomic forces are the most fundamental forces we are aware of today. However, I'm interested in considering some examples that are not so obvious.
Consider Hitler's invasion of France in 1940. As a result of the conquest, France lost control of its colonies, including Vietnam. Upon the end of WWII and France's renewed interest in taking back Vietnam, the people of Vietnam were no longer willing to be ruled by a foreign power. This is part of the reason Ho Chi Minh was able to solidify his rule in North Vietnam, and ultimately, all of Vietnam.
Now, the United States' involvement in the War of Vietnam was fueled primarily by the ideal of containment: to prevent the spread of communism. It's not so implausible to think that if Hitler had never invaded France, that the chain of events would not have transpired as they did. France never would have lost its colonies, and Ho Chi Minh never would have had enough support to lead his country in the direction of communism, making U.S. intervention superfluous. It is in this sense that Hitler may be considered one of the causes of the conflict in Vietnam.
*St. Thomas Aquinas