In 1898 Spain had no allies. They had adopted a foreign policy to not have a foreign policy. The "best foreign policy Spain can have is to have none." So no powers were obliged to come to their aid.
To come to their aid it would have taken a major world power. Spain had About 30 million people in 1898 with 10 million in its colonies mostly the Philippines and Cuba and these were both in near revolt. The US had 75 million people. Spain was producing about 2.7 million tons of coal per year. The US was producing 270 million tons. Spain was producing 166,000 tons of steel the US was producing 30 million tons.
Some military scholars of the time suggested Spain's superior military tradition would allow them to defeat the Americans. However, the vast majority saw the US as one of the worlds greatest industrial powers with ships guns and supplies far superior to the Spanish. Spain had only a single battleship (a battleship that had to refuel to make a voyage from Spain to Cuba) compared to the US navy's seven all of which could easily sail across the Atlantic and immediately begin operations.
The US had been experiencing a gradual warming of relations with Great Britain when the Spanish American War began. Dewey's fleet was cheered by British sailors as it sailed from Hong Kong harbor. Not to mention the coal which Britain sold to support the US fleet. Compare this with the Spanish. Their relief fleet to the Philippines was delayed in Egypt because they were unable to buy coal.
Communications sent by telegraph were officially neutral it was agreed that communications sent by land would be passed uninterrupted but that communications from ships to land would not be passed on without approval. The British who controlled most of the cables regularly broke this rule providing communication to US forces but not Spanish.
The US fleet in the Philippines was actually shadowed by a German fleet. The German fleet broke the US blockade and provided supplies to the Spanish in Manila. They only ceased provoking the Americans when Dewey threatened to fire on the more powerful German fleet. The Germans had hoped that a prolonged conflict in the Philippines would allow Germany to acquire the Philippines as a colony.
Similarly Russia also hoped to take possession of Spanish held Pacific Islands to use for refueling bases.
From the outset of the war it was clear to most nations that the US would likely emerge the victor. This meant that Spain not the US would require an ally. The allies who could have made a difference were Germany, Britain, France, Italy, Austria-Hungary, Russia and little known at the time but also Japan.
Germany was interested in trying to scoop up territories and may have sided with Spain for this goal if the conflict had continued long enough. Britain was pro-US and unlikely to support Spain. France required US coal and steel and was courting favor with Britain against Germany. They had no interest in chasing a conflict around the globe that would weaken their interests at home. Italy was still defining itself as a nation and could little afford a major conflict. Austria-Hungary didn't have the naval force to intervene in a conflict so far away. Russia was unconcerned except for their desire to acquire some refueling bases in the Pacific they saw the most likely path to this through neutrality and then bargaining with the victor. Japan also required US steel and coal and the US acquiring more routes to ship through would have helped this goal.
Basically the Nations of Europe were too afraid of each other to begin a war against the United States. They were also afraid that intervening for the US could lead to war against another European power.
Sources:
An Age of Neutrals Great Power Politics 1815-1914, Maartje Abbenhuis
The Encyclopedia of the Spanish American and Philippine American Wars a Political, Social, and Military History, edited by Spencer C. Tucker
European Perceptions of the Spanish American War of 1898 edited by Sylvia L Hilton; Steve Ickringill
War and Genocide in Cuba 1895-1898, John Lawrence Tone