The bulk of the collection consists of Clarke's handwritten (occasionally typed) letters to his wife Birdie. These letters are organized chronologically and date from April/May 1898 until July 1899, with a gap in June-November 1898 because Birdie was apparently in San Francisco with Clarke at that time.

The letters describe Clarke's impressions of the geography and cultures surrounding him, from his journey through the western United States to San Francisco, Hawaii, and the Philippines. He has a stereotypical white Euro-American regard for this era toward the Hawaiians, Japanese, Chinese, and Philippinos he meets. Terms and language used in his letters is representative of when he was writing and has not been replicated in this finding aid. Clarke expected some of his letters to be printed back home in newspapers in Lawrence, Kansas, so wrote detailed accounts of battles and what he saw on his travels.

Clarke discusses camp life, the other officers and "his men," studying Castilian Spanish, his activities as an officer (including drilling, serving as officer of the day, outfitting the men, food and water rationing, movements of various companies from across the United States, ensuring the soldiers got paid, etc.), military and political gossip, etc. Colonel Funston is one officer mentioned with some frequency. Other officers, including Funston, brought their wives to Manila Bay; Clarke disapproves and states that he will not ask Birdie to join him.

Clarke discusses his seasickness on the voyages to Hawaii and then to Manila Bay and otherwise mentions his health, as well as that of other soldiers; smallpox and malaria affected many soldiers. He also discusses the weather and social events and entertainments, including the Thanksgiving and Christmas holidays. Clarke frequently provides reassurances that he is well, requests that his family not worry, and describes his homesickness. He asks after friends and associates back home in Kansas, as well as providing directions for information to give them, and worries about his wife's health. He also discusses their future after the war, at times considering settling in Manila Bay, at times returning to Kansas, or remaining in the military. Clarke also provides lists of his expenses on a regular basis and lets his wife know when he is sending money home.

Clarke's company traveled to the Philippines on the steamer Indiana, arriving on the shores of Manila Bay in late November/December 1, 1898. He served on a court martial in January 1899, and his company saw battle in February and March 1899, "in the field beyond Caloocan." He was wounded in late March and spent April-July in the hospital and convalescing.

The collection also includes a few pieces of ephemera, mostly from 1898, and a few articles written by or about Clarke from later in his life after settling in Hawaii. There is also some secondary reference information about locations Clarke mentions in his letters and two letters from other individuals to Birdie Clarke.