On April 25, 1898, the United States officially declared war on Spain with the excuse of the explosion and sinking of the Maine . The military power of the United States, the obsolete Spanish fleet, the unstoppable Cuban independence process and the inexperience of Spanish politicians quickly tipped the balance... The Treaty of Paris of 1898, signed on December 10, ended the Spanish-American War or disaster on 98 and it put an end to the Spanish overseas empire... by means of said treaty, Spain abandoned its demands on Cuba and declared its independence; In addition, the Philippines, Guam and Puerto Rico were officially ceded to the United States for 20 million dollars.
USS Maine sinking
So where is the business? In the taking of Guam .
Guam , Guaján For the Spanish since the 16th century, by the end of the 19th century it was a forgotten possession with a small detachment of men protecting it. The last message that the Spanish authorities in Guam received from Spain was from April 14, 1898, a month before the conflict with the US, in which the possibility of a diplomatic rapprochement that would avoid an armed conflict was stated. Henry Glass , captain of the cruiser USS Charleston , was on his way to Manila when he received orders to take Guam. On June 20 he reached Guam and ordered three of his cannons fired, when the smoke cleared a small Spanish-flagged vessel approached the Charleston . The Spanish delegation asked to come on board to meet with the captain. To his astonishment, they greeted him effusively and excused themselves for not being able to return the salute... he had no gunpowder for cannon salutes . Glass informed him that war had been declared and that he was coming to take the island. 
USS Charleston
Due to the numerical inferiority of the Spanish garrison, scarcity of gunpowder and cannons - the only thing that was not in danger when firing it was for ceremonies -, without fortifications on the island, and without the possibility of help from the general Juan Marina , surrendered the island stating the following:
Without defenses of any kind, nor elements to oppose with probability of success to those you bring, I see myself in the sad decision to surrender, well protesting the act of force that is being verified with me and the way it has been done, I have no news from my Government that war has been declared between our two nations.
Following orders, the Americans flew the flag at Guam and continued on to Manila. Guam being the largest island of the Mariana Islands , and the most protected , it would have cost Glass nothing to take the rest... but in his orders nothing was said about it. With the loss of the Philippines, the administrative center of these territories, the archipelagos of Oceania became indefensible and ungovernable. So, in the interest of Germany, the German-Spanish Treaty was signed. of 1899 by which Spain sold the Caroline Islands and the Marianas – except Guam in US hands – to the German Empire for 25 million pesetas (17 million marks) .
Sources:The capture of Guam, Wikipedia