Пре­об­ра­зуй­те, если это не­об­хо­ди­мо, слово THEY так, чтобы оно грам­ма­ти­че­ски со­от­вет­ство­ва­ло со­дер­жа­нию тек­ста.

But the tradition of honoring those who died in defense of ______ country goes back to the Civil War. The tradition to hold memorial services, visit the cemeteries, decorate the graves with flowers and wreaths originated in Waterloo, New York, in 1866.