Given the following passage  "In contrast, the ROK Army defenders were relatively unprepared and ill-equipped. In South to the Naktong, North to the Yalu (1961), R.E. Appleman reports the ROK forces' low combat readiness as of 25 June 1950. The ROK Army had 98,000 soldiers (65,000 combat, 33,000 support), no tanks (they had been requested from the U.S. military, but requests were denied), and a 22-piece air force comprising 12 liaison-type and 10 AT6 advanced-trainer airplanes. There were no large foreign military garrisons in Korea at the time of the invasion, but there were large U.S. garrisons and air forces in Japan.",  answer the following question. Note that the answer is present within the text.  Question: what is the first time period mentioned?
1961