“让我们十分失望的是，战争最终在那两个国家之间爆发了。”翻译成英语。

To our's dispointment, the war broke out between the two countries at last.

Much to our disappointment, the war at last broke out between the countries.
对于这种句型：to one's + 抽象名词这种结构，用“非常、十分”等修饰时，一般习惯上用much放于最前面。
这个句子为了加强语气，最好把at last放在broke out之前。
用finally和eventually是不准确的。

这句记得昨天翻译过了。
To our great disappointment, the war finally broke out between the two countries.

To our's disappointment,the war broke out between the two countries eventually.

同学disappointment这个词你打错了吧

To our's dispointment,finally the war broke out between the two countries

Luogx101517的答案很好. 不过个人觉得NATIONS要比COUNTRIES好听点.