[1][2][3][4][5][6] From 31 July 1893 to 4 December 1894, Belknap served aboard the flagship of the Asiatic Squadron, the protected cruiser USS Baltimore.

When the Spanish–American War began on 25 April 1898, Newport was assigned to blockade duty at Mariel on the north coast of Cuba with Belknap aboard.

[4][7] On 14 May 1898, Belknap became the secretary on the personal staff of Commodore George C. Remey, commander of the naval base at Key West, Florida.

[1][4][6][7] From 1 April 1900 until 1901, Belknap had a second tour on the staff of George C. Remey—by now a rear admiral and the commander of the Asiatic Squadron—serving as aide with the duties of secretary aboard Remey's flagship, the armored cruiser USS Brooklyn.

During this tour, he took part in the Philippine–American War, served off the Taku Forts during operations in China in the summer of 1900 during the Boxer Rebellion, was promoted to lieutenant on 2 July 1900, and visited New Zealand and Australia on the occasion of the opening of Australia's First Commonwealth Parliament by The Prince George and Princess Victoria Mary of Teck, the Duke and Duchess of Cornwall and York, in May 1901.

In 1910, he served as special ambassador and naval aide to former President Theodore Roosevelt at the funeral of King Edward VII in the United Kingdom.

By mid-March, however, the compromise Belknap had brokered had collapsed, and the commander-in-chief of the Atlantic Fleet, Admiral Henry Thomas Mayo, reprimanded him for exceeding his authority in demanding that Menocal's forces not enter Santiago de Cuba.

In 1918, promoted to captain, he invented and patented a collapsible antisubmarine net and in the spring became commander of Mine Squadron One, which was tasked with laying the North Sea Mine Barrage, a primarily American effort to end use of the North Sea by German submarines as a transit route between Germany and the Atlantic Ocean by laying a dense minefield between the Orkney Islands and Norway.

After taking command, Belknap organized and trained the squadron, which was made up of ten large ships capable of carrying a combined total of 6,000 naval mines.

The war ended on 11 November 1918 before Mine Squadron One could finish a complete antisubmarine barrier, but the Barrage nonetheless is credited with sinking at least three German submarines and perhaps three more, as well as damaging three or four others.

[1][14][15] In 1925, Admiral Henry Thomas Mayo, who had been the commander-in-chief of the Atlantic Fleet in 1918, said: "The Navy and our country owe to Captain Belknap a debt which can hardly be over-estimated; for it was the knowledge and experience acquired and the doctrine and methods established in the Mine Force under command of Commander [sic] Belknap that enabled the Navy to, first, fit out improvised by very efficient minelaying vessels and, second, to operate them under war conditions in a manner which brought commendation from all.

[1][11][17] Belknap was transferred to the retired list on 26 June 1926, but remained on active duty, and on 3 May 1927 was promoted to rear admiral by an act of the United States Congress for his World War I service.

[8][11][17][19] In 1919, Belknap received the Distinguished Service Medal for his achievements in the North Sea Mine Barrage effort the previous year.

When Amy Phipps Guest offered to sponsor the first solo flight by a woman across the Atlantic Ocean, publisher George P. Putnam joined the search for a female aviator to undertake the journey.

[21] In 1910, G. P. Putnam's Sons in New York City published Belknap's account of his 1909 relief work in Italy, American House Building in Messina and Reggio.

In 1920, the United States Naval Institute in Annapolis, Maryland, published his book The Yankee Mining Squadron about the North Sea Mine Barrage operations he commanded during World War I. Belknap also wrote Introduction to the Life and Letters of Rear Admiral George Collier Remey, U.S.N., 1841–1928, published in Washington, D.C., in 1940.

[1] Averill Belknap married Andrew Robert Mack (1896–1977), a U.S. Navy officer who eventually attained the rank of rear admiral.

Their son Robert Belknap Mack also became a U.S. Navy officer and reached the rank of lieutenant commander before he was lost at sea on 24 September 1957.

[1] Emilia Belknap married Leonard Baker Cresswell (1901–1966), a United States Marine Corps officer who eventually attained the rank of major general.