Why did the US have a two year undeclared naval war with France?

The Quasi-War with France (1798-1800) grew out of disputes with the French as the Directory took power after the French Revolution.

In 1796, the US was negotiating trade with France's enemy, Great Britain, and had stopped paying its war debts to France after Louis XVI was dethroned. The French Navy began intercepting the cargoes of US ships trading with Britain. This was further aggravated by the XYZ Affair, in which French officials solicited payments from the US, essentially bribes, in exchange for stopping the attacks.

On July 7, 1798, after more than a year of piracy, and attacks on more than 300 US ships, the US rescinded its treaties with France and took steps to rebuild the US Navy. On July 9, Congress authorized the Navy to attack French warships.

Over the course of two years, the French captured hundreds more American merchant vessels, but 93 armed French vessels were captured or sunk by the US Navy and the Revenue Marine (later the Coast Guard). The war ended when Napoleon assumed power as Consul in France, with the signing of the Convention of 1800.