The Marseilles Pact, also known as the Imperial Alliance and the Mediterranean Accord was an alliance created in 1843 by France, and ultimately expanded to include the "associate members" of the empire, such as Italy, Poland, the Confederation of the Rhine, Netherlands, as well as Russia and Austria-Hungary. Although the United States of America later joined the pact in 1849, it was only on the condition that France would support the US, and vice versa, if the United Kingdom attacked the other, although both sides knew that they would come to support if they made the first strike. The primary reason for the alliance in Europe was to present a unified front (although overwhelmingly French) against both Prussia and the UK.
The alliance, however powerful, was not able to stand the strain of the First Global War, and since the US was busy trying to defeat the Confederate States of America, and, technically, Great Britain and America was not at war. The alliance dissolved in 1863, and it wasn't until Emperor Louis that the alliance was reborn as the Grand Alliance, in 1875.