I'm sorry, but do you have any idea how war spreads? Suppose Israel and Palestine are at war. For argument's sake let's say Egypt is enraged about the plight of Palestine and joins in on their side. Israel gets the equivalent support from, say, Greece. Now instead of 2 states at war, there are 4. And all you can ever accomplish by taking sides is to pile up more and more participants until you got yourself the next World War.
The most that outside states can accomplish is not get dragged into the war. They can't end the war, because to do that the warring sides first have to want it themselves. Which they clearly don't since they don't even recognize each other.