The United States sent money and weapons to Panamian Rebels because the Columbian Senate failed to ratify the Hay-Herrán Treaty. In exchange for money, the treaty was supposed to grant the United States the use of the Isthmus of Panama. This is a waterway that connected the Atlantic and Pacific Oceans.
The Columbian Senate did not want to lose their sovereignty. As a result, Theodore Roosevelt gave an approval to nationalist rebels to remove trains from the station of Colón and provided them with enough resources for the operation. This event stranded the troops of Colombia and discouraged them from marching towards Panama.