The ceremony was held in honor of those who had served their country in the military? 請問這句的in honor of 為什麼要放在句子的這個位子,文法是什麼?

The ceremony was held in honor of those who had served their country in the military.
in honor of those ...是一個「介系詞片語」（片語介系詞＋代名詞），用來當作述部was held的「修飾語」，屬於「副詞」性質。who had served their country in the military是「形容詞子句」，用來當作代名詞those的「修飾語」。
較長的述部修飾語，通常放在句尾，因為這樣比較自然，不會讓句子頭重腳輕。

The ceremony (in honor of those) served in military.
(in honor of those)--------domonstrative adjective phrase.qualifying the subject "ceremony".So it is placed after the subject.