We protested in Canada, and Canada subsequently did not join the war. Protest works, but there were flaws in that concept that doomed it to failure in the more militant countries. For example, people went out for one day then patted themselves on the back and went home. Mass protest activism needs to be sustained and escalate over time to be effective.
The Occupy protests were slightly more effective, in that they went on long enough to sway politics in the centre-right Democratic Party ever so slightly to the left. Now we at least get to see progressive voices like Elizabeth Warren in the news. It's not much, but it's better than nothing.
In the case of Palestine / Israel, public opposition has resulted in a barely perceptible wavering in the previously staunch "anything goes" and "our bank account is your bank account" approach US governments in both parties had toward Israel. Granted, there are other factors involved, for example the fact that the US is broke, but this is the first time in (my) living memory the US has considered attaching any conditions at all to their military aid to Israel.