What law passed to keep the United States from being drawn into war?
Not sure what time frame you mean either, but some other possibilities would include George Washington's Neutrality Proclamation, the Embargo Act under Jefferson and the Non-Intercourse Act under James Madison.
Washington was aiming at keeping us out of alliances between empires that might draw a new and weak nation into conflict. Future Presidents often times followed this tradition. Jefferson was trying to ensure that we did not become involved in the Napoleonic Wars so the law Congress passed prohibited trade with any country fighting in that war. Madison realized the law was unpopular and modified it with the Non-Intercourse Act which prevented trade only with Britain and France instead of all of Europe. Both of those laws were intended to keep the US neutral.
There have been various laws passed that have been meant to keep the US from being drawn into war, but I think what you are most likely to be talking about is the Neutrality Acts of the 1930s.
In World War I, the Germans blockaded England using submarines. US ships were put in danger of being sunk as they tried to bring goods to England (just regular trade, not even weapons). The US Congress decided that one thing that got us into WWI was trading with a country that was at war. So the Neutrality Acts prohibited lots of kinds of trade with countries that were at war.
If this is not the right time period, let me know via message...