Yes.
"And on 1967, the Arabs led by Egypt under the rule of Gamal Abdul Nasser, went in war with Israel and lost more Palestinian land and made more Palestinian refugees who are now on the mercy of the countries that host them."
More: Arab Spring and the Israeli enemy
at someone's mercy
Can I use the preposition on with the underlined idiom instead of the preposition at? Thanks in advance.
at the mercy of
at the mercy of
be at the mercy of sb/sth
Yes.
"On the mercy of" sounds weird to me. As does "went in war with."