It is best known for causing all sorts of chaos in the Middle East, but today it has an extensive presence in Europe, Africa, Latin America and North America. Hezbollah has made billions selling drugs inside the United States, and at one point they had more than 70 used car dealers in this country through which they were laundering drug money. Of course the primary source of funding for Hezbollah is Iran. The terror organization was founded in 1985 with heavy support from Iran, and according to Wikipedia "most of its financial, training, weapons, explosives, political, diplomatic, and organizational aid" currently comes from the Iranians. In essence, Hezbollah is Iran's terror arm, and so if we go to war with Iran we will find ourselves in a conflict with Hezbollah as well.
Most Americans have no idea what a full-blown conflict with Hezbollah could look like. For a moment, just imagine thousands of extremely well-trained terrorists shooting up and blowing up soft targets all across America. The sheer panic that would cause would immediately paralyze our entire society.