I haven't read too much about the subject, and I invite anyone that knows more than I do to correct or amend my answer. Most of the information is synthesized from the SEP article on coercion, but as always, I suggest reading the primary sources.
To answer your question, you first need to precisely define coercion as things are not as simple as they appear. There are, generally speaking, several schools of thought regarding coercion, purported by Aquinas, Hobbes, Locke, Kant, Mill, and Nozick.
Aquinas - The 'inclination of the will' and non-coerciveness of threats
For Aquinas, the result of coercion is the disconnect between what he calls the 'inclination of the will' and the action taken by the agent. Simply put, it looks something like this:
- I want to go to the store -> [no coercion] -> I voluntarily go to
- I want to go to the store -> [coercion] -> I involuntarily do not go to the store
The first column being the 'inclination of the will.' It's also of note to say that, according to Aquinas, in case
A, the agent is responsible for his actions, whereas in case
B, he is not. Lets go back to your original question; if you agree with Aquinas, you don't really 'influence' anything, anyway. It's as if you make the coercee your puppet (you control them but 100% of the responsibility falls on you).
"But how far can we go without coercing?" you may ask. Aquinas makes a very interesting distinction. He argues that anything non-violent cannot be construed as coercive. So you could technically threaten, terrorize, or otherwise non-violently 'influence' someone to do your bidding and Aquinas would say two things: a) it's not coercion (yay!), and b) the agent is, in fact, 100% responsible for the actions they may take.
- If you don't vote for me, I'm going to kill you.
- I will rape your wife unless you give me your wallet.
- Every second you don't sign the contract, your company loses $1,000,000.
Are all not non-coercive according to Aquinas.
2 are threats and Aquinas doesn't believe threats can possibly be coercive.
3 is not a threat (your company is losing money as we speak!) but it's non-violent so, again, it's not coercive.
If you run a government and subscribe to this kind of philosophy, you're in luck. You can do just about anything. Violent torture, punching someone in the face, or otherwise being violent while attempting to undermine someone's 'inclination of the will' is the only way of being coercive. Raising gas prices is the least of your worries. Primary source reading: Summa Theologica.
Hobbes, Locke, Kant, Mill - Justice and the role of the state
Hobbes, Locke, and Kant mainly thought of coercion as a tool of the state that ought to be conducive to justice. Even Aquinas thought that a governor needed certain coercive powers to ensure order. Hobbes, in particular, thought it to be an essential part of government:
[W]here there is no coercive Power erected, that is, where there is no
Commonwealth, there is no Propriety; all men having Right to all
things: Therefore where there is no Commonwealth, there nothing is
Unjust. So that the nature of Justice, consists in keeping of valid
Covenants: but the Validity of Covenants begins not but with the
Constitution of a Civil Power, sufficient to compel men to keep them:
And then it is also that Propriety begins (Hobbes 1651, Ch. 15).
All four (also Bentham and others) believed, like Aquinas, that a 'covenant from fear was valid,' so fear is, therefore, not coercive. The limits of coercion seem to align closely to what Aquinas laid out. Mill, however, argued that coercion consists of more than violence and threats thereof. He made several leaps:
- Social institutions (not the government) possess coercive methods
that are much more powerful than the state's. He cites marriage as
- Fines, laws, taxes, etc. are all instances of coercion. He doesn't say these things are bad per se, but they are coercive.
If you agree with Hobbes, Locke, and Kant, you're more or less golden as their views on what is and is not coercive agree with Aquinas'. You're not coercive until you have to violently impose something on someone.
Mill is trickier. Even having fines for say, littering, implies coercion. Of course, he argues that it's necessary, but it's coercive nonetheless. It would be difficult to influence anyone one way or another without any leverage. Primary source reading: Mill's On Liberty.
Nozick - Coercive threats and (no) unsuccessful instances of coercion
Nozick says a lot about coercion and I would do you a great disservice if I tried to simplify his position here. Parts of his argument that I think are relevant (and revolutionary) are the following:
- Threats can be coercive.
- An instance of coercion occurs if and only if the coercee does what she is told to do.
So, according to Nozick, you can influence anyone insofar as you don't threaten them (or obviously don't actually violently abuse them) IFF the threat leads to their action being dictated by you.
It seems like a silly thing to say, but it makes a lot of sense. Check out this simple thought experiment.
You run a country but hard times fall on it. Crops die out, many are
emigrating, etc. You are forced to pass austerity measures that many citizens aren't fond of but you're doing your best and you've always been a fair
leader (a). However, there's a small dissident group forming (lets call them
the Rebels). You eventually capture their leader, Bob. Bob is a
zealot. He truly believes that you're a horrible leader. First you try
to have a conversation with him. You try to reason with him, you try
to make him understand that it's hard being in power and that not
everything is black and white. As mentioned, he's a zealot. He rejects
any rational argument, so you move to the next step. You torture him
and attempt to force him to renege his allegiance to the Rebel force,
but no dice (b). He's convinced he's right. The next morning, you decide
to do the next best thing: you publicly execute Bob. The Rebellion
soon dies as many fear and decide to sever their ties with the
Primary source reading: Socratic Puzzles.
Conclusion - How far is too far?
Looking at the example above, we see various milestones of coercion:
- According to Mill, you coerced citizens with austerity measures in
- According to Aquinas, Hobbes, Locke, and Kant, you coerced Bob with torture in (b).
- According to Nozick, you coerced the Rebel onlookers in (c).
So, the answer your question of "how far can you go," depends on two things: 1) how, exactly, you define coercion and 2) the potential coercee's willingness to change their mind.