Questions: do you believe 1-3 are succesful? Have you ever seen them tried out?

Answer immediately.

3 has been very successful, but it usually ends in a stab as well. You both kinda know its coming though.

1 is absolutely pointless. Better to keep them thinking you're on the same page.

2....2 usually doesn't end the alliance if its done right. Depending on the circumstances, they may just join you in the attack, so I don't think it would end the alliance in most cases

How does 2 not end the alliance? You just ended it.

I always tell them a turn in advance, its the only honourable thing to do...

Ah, I misinterpreted it. I thought, say both England and Italy were friends with Russia, heading for a 3-way. Italy stabs Russia (former ally) so then what does England do?

I agree with goldfinger on this one

1. Might as well just keep them thinking your happy and cool with your alliance.
2. I believe if you're in an alliance and than you feel like going for the 18th you just tell him like ''lets fight it out'' i believe this is only valid when he has contributed significantly to your partner ship if its been more of a one sided alliance where you have done most of the work stick with 1.
3. Goldfingers is right there is no real way there can be a race to 18 without the inevitable stab.

Thanks guys, I'll remember this.
@goldfinger, no it was more the A and B were allied and then A sends a message to B that declares war, do you think it's better/safer/diplomer?

# 1 is the only answer. Option #2 is a stab, and #3 requires a stab.

No, I never formerly declare war (unless I'm in a very pompous mood and my victory is assured). That gives them time to construct alliances against me and everything. No, much better to just slip the knife in quietly. So in answer to your response, it may make you feel better on the inside, but as far as the game goes it is not safer and not diplomer. It's much more impressive to pull off the good stab or even better, convince your enemy that the stab is necessary (after the fact of course).

Of course it depends on the situation. Say Russia is fighting Turkey and Italy, has killed off Austria already (ie. He's huge). You're England and you want Turkey to survive as long as possible, so you go ahead and tell Russia you're stabbing. Now if this pulls back some of his units from the southern fronts and relieves some pressure off of your allies, then you've got a diplomatic success there. So in general, everything's an option depending on the scenario. It's just that this option in particular has a very narrow set of scenarios it's good for

^I did that once and it worked out very nicely. I declared war on a former ally (this was on playdip) and then he withdrew all his units to me. He then furiously asked me "why I wasn't sending any troops". haha gotcha. :)

Declaring war has it's uses. If you're in a situation where war with a neighbour is so obviously the right thing to do that any idiot could spot it a mile off, you might as well let him know in advance and wish him good luck in what you hope will be an enjoyable fight. That way when you have taken the SCs you wanted there is a much higher chance that those that remain will not dedicate the rest of the game to making your life a misery. He may even help you out if he hates the others enough.

^Yeah that was "in theory" what I hoped for.
How about
4) mutually agreeing on starting a war. Simply proposing to the other player if he also feels now is the time to start trying for solos?

all sounds stupid to me except for some scenario's as mentioned above...
just stab!
also, in the scenario with Russia you don't even need to attack, you just declare war and he moves north and after 1 or 2 phases you propse peace, telling you just wanted to give Turkey/Italy a chance, he has no chance to attack England well because he's the dude with at least 2 fleets swimming around and having 2 northern fleets as well is kinda stupid...

in public it's pretty much the same only is the impact way greater and you could declare war on someone just to get allies and then make peace again and then you're the neutral, always nice being neutral...