When is a country allowed to intervene in another country's affairs?

In general, most countries follow the rules of international law and diplomacy which require that no country interferes in the affairs of another. Not at all. Ever.

But there are exceptions. Particularly if one rogue country is breaking the rules itself and invading another or causing harm to another. In such cases there are international courts which should be approached for rulings, as well as the United Nations.

If such courts and the UN decide to take action, it is usually graduated action, slowly ramping up through through diplomatic approaches, trade embargoes, total blockades and eventually armed intervention - also called War. There is also an exemption considered under humanitarian law, allthough not binding, that considers the case when the population or large sectors of the population of a country are at risk of life because of lack of centralized government or lack of action from a central government. In such extraordinary situation, a non military action may be taken by a coalition of at least two countries under the supervision of the United Nations for relief or evacuation purposes. An extreme danger to the nationals of a foreign country who have residence or are in transit in a given country, may also grant a rapid intervention from their country of origin or citizenship, with the exclusive purpose and limitation of evacuation. Nevertheless, the above are extraordinary exceptions that may cause further instability or unwelcome consequences, hence the primary international binding to the requirement that no country may interfere in the affairs of another at any time.