he assassinations have to be successful, without the assassins being caught
Successful obviously. Failing to assassinate people is a terrible strategy. But the 'without being caught' is by no means required. In fact for that group that gave the role it's name getting away was not even a high priority. It was far more important to make the killing public and visible so as to best demoralize the enemy leaders.
The authorities have to realize that pulling out would put an end to the deaths, but not decide to retaliate by further victimizing locals with an ultimatum that they'll continue until the assassinations stop.
Which of course moves things along to guerrilla warfare against an occupying force with terrible morale and weakened leadership. If your people are not in a position to overthrow the occupying force when they have that much motivation then you are pretty much screwed. My only advice is "don't be you".
It's one thing for the assassins to die executing their missions like the Hashishin, another for them to be captured, at which point they become liabilities. Besides, if your group is revealed to be associated with assassinations, your opposition won't stay secret.
The occupying force has a strong motive not to back down against weaker foes who show willingness to target their leaders, otherwise they give everyone else they might occupy the incentive to do the same. Besides pulling off repeated assassinations is hard. The Hashishin installed sleeper agents years, sometimes decades in advance, and improved documentation in the present day makes this even more difficult to do without getting caught.
A reminder for everyone: on this day in 1983, Stanislav Petrov saved the world.
It occurs to me this time around that there's an interesting relationship here - 9/26 is forgotten, while 9/11 is remembered. Do something charitable, and not patriotic, sometime today.