the reasons that they participate in the war are not similar.請問這句話有沒有不通順的地方?

The reasons that they participate in the war are not similar. (OK)
The reasons why they participate in the war are unique on their own right. (better)

Their "opinions" on the matters of war are "different."
They have "not similar=unlike" reasons to participate in war.
Their participation in war are "not similar"(are not the same,are dissimilar, are "differential=showing a difference", are difficult to tell)

通順的說就是:
They have different reasons to participate in the war.

[The reasons that they participate in the war] are different. 華語譯：他們參加戰爭的原因是不同的。
# similar 用於同類的東西而其性質、形狀等相似。
# different，distinct：強調個別性。

Amendment: The reasons for them waging in wars were not the same. Yip