Roosevelt resigned his post in 1898 to volunteer to serve in the Spanish–American War of 1898. He entered as a lieutenant colonel under the leadership of Colonel Leonard Wood, his friend who had earned the Medal of Honor for his combat record. Roosevelt trained with Wood’s men at San Antonio. His so-called “Rough Riders” were a tough military unit that later achieved great success during the war, particularly at the Battle of San Juan Hill near Santiago, Cuba, when Roosevelt was a full colonel and personally led his troops into combat. He left the conflict as a genuine military hero. He later wrote a book about his experiences in Cuba, aptly titled Rough Riders.