Obverse side of War flag of Siam (now Thailand). This flag was designed by King Vajiravudh (died 1925) for encouraging Thai troops who fought with Allied Power against the Central Powers during World War I. The obverse of the flag is 2:3, like the modern flag of Thailand (Triranga) but defaced with a red circle. The circle edge coincides with the inner rim of the red stripes. In the center of red circle, there is a dressed white elephant. The elephant is facing the flagstaff. This flag is not in use now.The inscription on the flag is from a poem written in Pali, (assumed) to have been 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