The United States is not party to the war between the European powers. However, despite its neutrality, American industry is happy to sell war materiel to anyone who will pay for it. The British blockade prevents the Germans from importing arms from the USA, so in practice it is only the Allies who are able to supply themselves with American arms.
It irks the Germans and Austro-Hungarians that the United States is arming their enemies. Their U-boats are intercepting some of the ships carrying war goods and perhaps will catch more once unrestricted submarine warfare commences.
To disrupt America’s arming of the Allies, the Central Powers have also resorted to more underhand efforts. Saboteurs are operating in the United States, trying to start fires in American munitions factories and storage depots.
Today a fire erupts in a munitions factory in Kingsland, New Jersey. The plant is destroyed, together with as many as half a million artillery shells. The cause of the fire is not conclusively established and may conceivably be accidental (explosives are prone to explode, after all). As with the Black Tom explosion however, suspicions fall upon the agents of the Kaiser.
inspecting the damage (Wikipedia)