Late arrival to this posting, but I think there a a further couple of points worth making - whilst still agreeing wholeheartedly about not starting a feud that could simmer for years.
We saw and bought our house with vast valley views and did note the walnut tree that sat in the middle of it, but which we also noted provided our mitoyenne (holiday home) French neighbours with the only shade they have. They are lovely people and did suggest that as it was affecting our view they might cut it down. Two things applied here, one I have an aversion to cutting down healthy trees unless absolutely necessary, and two, as the trees are decidous in the winter still mild months the leaves fall so we still can enjoy the view. We have no money available for forestry work, and are both pensioners at at 76 years old I have not the slightest intention of climbing up ladders with power saws to trim or otherwise attack the tree.
On the other side we have another tree that belongs to our neighbour who happens to be the Maire, and that has grown over the past four years to block permanently (as it is an evergreen) some 25 percent of our view. Just to compound the issue we have in our turn trees that have grown which block the valley view for the retired Abbé.
So here's a potential stew to upset a lot of people, but the answer has to be in compromise. Taking the last case first the Abbé approached us about his loss of view and we agreed that he could get our trees cut back whenever they became a problem. He was happy to pay for this. Obviously no problem for us. We have yet to approach the Maire about his tree, BUT in the interim we have had a young English couple with three young children move into an old ruin about four doors down. To be brutally frank about it so far they seem to have turned the ruin into a slum, so if the Maire's tree gets cut down (which we don't want) or even trimmed, we will simply get a 'better' view of the builder's yard slum, as the English guy has cut down all the trees on his property to get a better view! A clear case of vandalism for me, but its his land. So second situation resolved by evolving circs.
We will be asking for the walnut tree to be trimmed back a bit in width, and as it is non-productive, and a bit straggly we think this will produce an acceptable result all round.
So the point I am trying to make is that trees grow, circumstances change, and that all considerations should go beyond the here and now, and consider the future, both on the ecological sense and the personal relationships sense.Whatever you do, remember that it will take time to turn round any errors.
We noted that someone persuaded the old lady owner of a large property here that she was missing out of the view and that her magnificent hedge should be cut right back. She now has an uninterrupted view of the side wall of the house slightly lower than hers on the slope, but nothing more. She has lost all her own privacy and gained nothing in the view.
Finally one point about the relevance of the health of any trees on the properties - and where they are rooted I understand to be the point of responsibility. If a healthy tree is torn down by storms for example this is usually covered by household insurance, and even any damage done to power lines is not usually the householders liability, neither is tree removal across a road. However, it is my clear understanding that if the tree is seen to be rotten and had not been cut down by the householder any insurance is null and void and could mean facing substantial repair bills - I think the roads and power lines would not be the responsibility of the householder though. Worth checking the insurance on this. Also remember that removing a healthy tree can have an effect on drainage, so it is not something to be done lightly.