Question: In which military branch did the president during the purchase of the state where Platinum Airport is located serve?

full_context: Document 1: Manokotak Airport: Manokotak Airport is a state-owned public-use airport located one mile (2 km) north of the central business district of Manokotak, a city in the Dillingham Census Area of the U.S. state of Alaska.
Document 2: Lincoln (novel): Set during the American Civil War, the novel describes the presidency of Abraham Lincoln through the eyes of several historical figures, including presidential secretary John Hay, First Lady Mary Todd Lincoln, Secretary of State William H. Seward, Secretary of the Treasury Salmon Chase, his daughter Kate Chase, U.S. Representative Elihu B. Washburne, and conspirators John Wilkes Booth and David Herold.
Document 3: Department of Alaska: The Department of Alaska was the designation for the government of Alaska from its purchase by the United States of America in 1867 until its organization as the District of Alaska in 1884. During the department era, Alaska was variously under the jurisdiction of the U.S. Army (until 1877), the U.S. Dept. of the Treasury (from 1877 until 1879) and the U.S. Navy (from 1879 until 1884). The area later became the District of Alaska, then the Territory of Alaska, then the State of Alaska.
Document 4: Alaska Purchase: Russia continued to see an opportunity to weaken British power by causing British Columbia, including the Royal Navy base at Esquimalt, to be surrounded or annexed by American territory. Following the Union victory in the Civil War, the Tsar instructed the Russian minister to the United States, Eduard de Stoeckl, to re-enter into negotiations with William H. Seward in the beginning of March 1867. President Johnson was entangled in negotiations about Reconstruction and Seward had alienated a number of Republicans, so they believed that the purchase would help divert attention from the domestic issues. The negotiations concluded after an all - night session with the signing of the treaty at 04: 00 on March 30, 1867, with the purchase price set at $7.2 million ($105 million in 2016), or about 2 cents per acre ($4.74 / km).
Document 5: Alaska Purchase: The Alaska Purchase (Russian: Продажа Аляски, tr. Prodazha Alyaski) was the United States' acquisition of Alaska from the Russian Empire on March 30, 1867, by a treaty ratified by the United States Senate, and signed by President Andrew Johnson.
Document 6: Platinum Airport: Platinum Airport is a state-owned public-use airport located in Platinum, in the Bethel Census Area of the U.S. state of Alaska.
Document 7: Catharinus P. Buckingham: Catharinus Putnam Buckingham (March 14, 1808 – August 30, 1888) was an American soldier, college professor, author, and industrialist. He served as a general in the Union Army during the American Civil War, and was the main assistant to the U.S. Secretary of War, Edwin M. Stanton, during part of the first term of the Lincoln Administration.
Document 8: John Aaron Rawlins: John Aaron Rawlins (February 13, 1831 September 6, 1869) was a general officer in the Union Army during the American Civil War and a cabinet officer in the Grant administration. A longtime confidant of Ulysses S. Grant, Rawlins served on Grant's staff throughout the war, rising to the rank of brevet major general, and was Grant's chief defender against allegations of insobriety. He was appointed Secretary of War when Grant was elected President of the United States.
Document 9: Alaska: Many communities have small air taxi services. These operations originated from the demand for customized transport to remote areas. Perhaps the most quintessentially Alaskan plane is the bush seaplane. The world's busiest seaplane base is Lake Hood, located next to Ted Stevens Anchorage International Airport, where flights bound for remote villages without an airstrip carry passengers, cargo, and many items from stores and warehouse clubs. In 2006 Alaska had the highest number of pilots per capita of any U.S. state.
Document 10: Reconstruction era: As Confederate states came back under control of the U.S. Army, President Abraham Lincoln set up reconstructed governments in Tennessee, Arkansas, and Louisiana during the war. He experimented by giving land to blacks in South Carolina. By fall 1865, the new President Andrew Johnson declared the war goals of national unity and the ending of slavery achieved and reconstruction completed. Republicans in Congress, refusing to accept Johnson's lenient terms, rejected new members of Congress, some of whom had been high - ranking Confederate officials a few months before. Johnson broke with the Republicans after vetoing two key bills that supported the Freedmen's Bureau and provided federal civil rights to the freedmen. The 1866 Congressional elections turned on the issue of Reconstruction, producing a sweeping Republican victory in the North, and providing the Radical Republicans with sufficient control of Congress to override Johnson's vetoes and commence their own ``Radical Reconstruction ''in 1867. That same year, Congress removed civilian governments in the South, and placed the former Confederacy under the rule of the U.S. Army. The army conducted new elections in which the freed slaves could vote, while whites who had held leading positions under the Confederacy were temporarily denied the vote and were not permitted to run for office.
Document 11: Alaska Purchase: The Alaska Purchase (Russian: Продажа Аляски, tr. Prodazha Alyaski) was the United States' acquisition of Alaska from the Russian Empire on March 30, 1867, by a treaty ratified by the United States Senate, and signed by president Andrew Johnson.
Document 12: Alaska: Regular flights to most villages and towns within the state that are commercially viable are challenging to provide, so they are heavily subsidized by the federal government through the Essential Air Service program. Alaska Airlines is the only major airline offering in-state travel with jet service (sometimes in combination cargo and passenger Boeing 737-400s) from Anchorage and Fairbanks to regional hubs like Bethel, Nome, Kotzebue, Dillingham, Kodiak, and other larger communities as well as to major Southeast and Alaska Peninsula communities.
Document 13: Alaska Purchase: Russia continued to see an opportunity to weaken British power by causing British Columbia, including the Royal Navy base at Esquimalt, to be surrounded or annexed by American territory. Following the Union victory in the civil war, the Tsar instructed the Russian minister to the United States, Eduard de Stoeckl, to re-enter into negotiations with William Seward in the beginning of March 1867. President Johnson was entangled in negotiations about Reconstruction and Seward had alienated a number of Republicans, so they believed that the purchase would help divert attention from the current domestic matters. The negotiations concluded after an all - night session with the signing of the treaty at 04: 00 on March 30, 1867, with the purchase price set at $7.2 million ($123 million today), or about 2 cents per acre ($4.74 / km).
Document 14: Tuntutuliak Airport: Tuntutuliak Airport is a state-owned public-use airport located one nautical mile (1.85 km) south of the central business district of Tuntutuliak, in the Bethel Census Area of the U.S. state of Alaska.
Document 15: Lafayette C. Baker: Lafayette Curry Baker (October 13, 1826 – July 3, 1868) was a United States investigator and spy, serving the Union Army, during the American Civil War and under Presidents Abraham Lincoln and Andrew Johnson.
ground_truth: Union Army
sub_queries: Which state is Platinum Airport located? Answer: Alaska
who was the president when Alaska was purchased Answer: Andrew Johnson
Andrew Johnson >> military branch Answer: Union Army