Reverse side of War flag of Siam (now Thailand). This flag was created by King Vajiravudh (died 1925) for encouraging Siamese troops who fought with Allies Power against Central Power in World War I. This side of the flag is 2:3 as Triranga but defaced with red circle. The circle edge coincides with inner rim of red banner. In the center of red circle, there is golden glowing Great Victory Crown. Below the crown, there is personal emblem of King Vajiravudh. This flag is not in use now.The inscription on the flag (the same on both the obversen and the reverse) is from a poem written in Pali, (assumed) to have be written by a Thai monk in the 6th century AD, entitled Buddha Jayamangala-gatha, or Chant to the Buddha's victories. The text, in Thai, and transcribed is: 




พาหุํสหัส์สมภินิม์มิตสาวุธน์ตํ,
ค๎รีเมขลํอุทิตโฆรสเสนมารํ.
ทานาทิธัม์มวิธินาชิตวามุนิน์โท,
ตัน์เตชสาภวตุเมชยสิทธินิจ์จํ.





Bāhuṃ sahassam-abhinimmita sāvudhan taṃ,
grīmekhalaṃ uditaghora sasenamāraṃ.
Dānādadhammavidhinā jitavā munindo,
tan tejasā bhavatu me jayasiddhiniccaṃ.



The entire poem can be read on Thai Wikisource: พุทธชัยมงคลคาถา (modern orthography).

Thanks for original image from Thai Heritage Museum and Xiengyod's Site([1], [2]).
Own work
Tangmo