What stops countries from acting unilateraly

Question

There is a kind of similar question at Why not leave UN.

Let's suppose, that leaving UN and other treaties signed by the country is perceived an extremely negative option.

But what, if a country have an unresolved dispute with its neighbors (pick anyone you like/dislike the most), then due to the dispute the internal politics is in favor of changing the regime and the newly formed nation rejects any legal continuity with its predecessor.

At that point a new country, even if not recognized de jure, could in fact act without previously binding limitations. And in some fantastic outcomes resolve the long-standing dispute. Even by force, which is condemned by everyone.

At that point the country could be deemed to be pariah, and this is bad (for them).

Then a new country could be formed, that announces its continuity with the original state and denies the wrong-doings of the second one, but fully enjoys the new de facto resolved dispute state.

What prevents countries from going this path?

Answer 1

What prevents countries from going this path?

Other countries. Namely, their military and economic influence. If other countries will fail to prevent this, no one else will (literally).

Leaving the UN isn't really an important part here:

• Russia annexed Crimea and supports military unrest in Eastern Ukraine.
• USA-led coalition straight-up conquered Iraq.

Both of these countries are permanent members of the UN Security Council.

Resolving conflicts through a military force doesn't really disqualify the country from the membership in the UN. Your hypothetical country (let's call it Hyponia) doesn't need to leave the UN to pull this through. In fact, it would be unwise for it to leave the negotiating table during the whole process.

Here's your scenario:

... the newly formed nation rejects any legal continuity with its predecessor. ...
... a new country ... resolve[s] the long-standing dispute ... by force, which is condemned by everyone.
Then a new country could be formed, that announces its continuity with the original state and denies the wrong-doings of the second one, but fully enjoys the new de facto resolved dispute state.

I can see two possible outcomes:

1. No one buys this whole "regime-change/new country" thing. Neighboring countries get the support of the UN Security Council and they spend next year or so, carpet-bombing Hyponia (ISIS scenario), destroying it completely.

2. No one buys this whole "regime-change/new country" thing. Neighboring countries fail to get the support of the UN because Hyponia has a powerful enough military/allies to stand their ground (Kosovo scenario) and/or the whole region is a big mess no one really cares about (Somaliland scenario). No one intervenes, Hyponia enjoys status-quo with some lazily-applied economic sanctions.

As you can see, leaving the UN doesn't make Hyponia immune to foreign influence.

Either way, if Hyponia was already the member of UN it doesn't make much sense for it to leave the organization. Actually, it isn't even necessary to rename itself or go through regime change. It's much simpler for Hyponia to unilaterally break away from all of the peace treaties and declare a war with its neighbors. All without leaving the UN.

Answer 2

What makes a country a country?

The fact that the citizens and other countries agree that it is a country. The idea of nationhood predates the codification of customary international law, not the other way around. Enough people agree that Canada or Chile are sovereign countries, for example. Other countries exchange ambassadors with them, their passports are accepted, and so on. Enough people disagree that Catalonia is a sovereign country, so their passports would not be accepted if they were to issue them, and so on. In a way sovereignty is a self-fulfilling prophecy. If enough people believe that a country is a country, then it is a country.

Being accepted as a member of the UN is a big step towards reaching this consensus that a country is a country. Consider how strongly the Palestinians fought for membership in various UN bodies. Or how the Federal Republic of Germany fought against any recognition of the German Democratic Republic.