You might find the answers in the linked document. Bottom line is that the legal basis for the occupation are mostly the Article 43 of the 1907 Hague Regulations and Article 64 of the Fourth Geneva Convention (1949 I belive). But you cant just read them in a strait way.There is much jurisprudence and those regulation have a very nuanced contemporary meaning.

The issue is quite complex but regarding Iraq if I may cite  from the document: " Thus, it might be argued that remodeling the core institutional structures of certain States – be they weak, in collapse or posing a threat to peace and international security – is a legitimate task ascribed to the occupying power. Nevertheless, the question arises whether “transformative occupation” has any basis in law."

https://www.icrc.org/en/publication/4094-occupation-and-other-forms-administration-foreign-territory-expert-meeting

Thank you, this is a really interesting source ... So the legal character of these intervention is seen controversial ... And the assertion of the legality seems to be, in the long run and the last instance, a question of power. 

I see it as a process of limiting power. Sure, some regulation might be inadequate but there is a growing consensus that there is a binding and relevant set of rules that emerge, albeit slow. You can see it in actions of invading countries. More often then not their are seeking some link to this rules rather then ignore them. Even if they have the power they often go through some sort of legal dispute ( often flimsy) to legitimize their actions.  This is because common international consensus has a legality effect and the recurse to pure power and domination is not sufficient. But the core of law is that behind it is some sort of state power to enforce it ( what is Justicia/Temida holding beside the scales?) And lack of any one authority in international law makes it a bit different, more soft one could say.

the controversial R2P (Responsibility to Protect) and Chapter VII of the UN Charter could be some legislative provisions for (military) intervention and subsequent regime change. However both are disputed and criticism against UN is mounting. It depends which Weltanschauung-lense you apply. Good luck!

I don't know much about international laws and rules for legitimizing foreign occupation in the name of peace and democracy; but as a layman I do know is the UN Security Council can be easily used by a cohort of military , corporate powers to shed blood in any foreign land by securing a resolution.  

Firstly it is vial to ascertain the cause of occupation to evaluate its legitimacy. Second imperative facet is the native opinion about policies of occupants. If cause of occupation is considered justified by the occupants and they like occupant's policies than it only being a notion of positive morality is enough and in case it is other way round then it will be according to International law of MIGHT IS RIGHT. 

Role played by UN in uni-polar world has has further weekend the influence of International Law.

If Security Council approves a resolution about intervention, it is legal. The basis is UN Charter and according to the Carter only Security Council is responsible for such decisions. This is the reason (and argument) why  Russia and some other countries criticized military activities of NATO in former Yugoslavia, Iraq 2003. Afghanistan case was sanctioned by SC.

Gulf War 2? That's the point that a country and its proxies on the basis of their  interest can either veto or support the killings in the peace and democracy.  With such a club of self seeking powers, discussion about international laws about intervention, or invasion or intrusion is only but academic.

A possible answer which may indirectly throw some light on the challenging question of «foreign military occupation with subsequent regime change” is to consider the advantages and disadvantages of various alternative mechanisms of regime change, and contrast these with the advantages and disadvantages of the current states of affairs with challenging legal issues. That’s the approach in the following paper, and the references may possibly be useful.

Hausken, K. and Knutsen, J.F. (2010), “An Enabling Mechanism for the Creation, Adjustment, and Dissolution of States and Governmental Units,” Economics: The Open-Access, Open-Assessment E-Journal, Vol. 4, 2010-32. doi:10.5018/economics-ejournal.ja.2010-32, http://dx.doi.org/10.5018/economics-ejournal.ja.2010-32.

http://dx.doi.org/10.5018/economics-ejournal.ja.2010-32

Article An Enabling Mechanism for the Creation, Adjustment, and Diss...

The 2003 War on Iraq, this war should never have been embarked upon until the alternatives had been exhausted. In other words, when the means of diplomacy get exhausted, the voices of war rise. On this account, any action in war is impermissible without a legitimate reason. Thus, it should take a war resolution through the international consensus via the UN Security Council, respecting its international prestige.  

Dear Holm

You can find an evidence about this vital issue in this link which gives you all answer that you looking for. 

http://www.iraqinquiry.org.uk/the-report/

This report which published before one month it will be the most important documents about this unfair war .

R2P is the only mechanism I can remember, which is enforced only when there's reasonable evidence of systematic violation of the human rights of a defenseless civilian population 

Dear Holm, 

from my perspective the above mentionned R2P issue is crucial. 

In order to judge the legality or illegality of of intervention I do not see an all-seize-suits-all answer. 

Even the R2P concept leaves a wide space for interpretation and hypocrisy. 

As you refer to Afghanistan: 

What I found very relevant as practitioner in Afghanistan was the following input: 

 Cockayne, J., & Lupel, A. (2009). Conclusion: From Iron Fist to Invisible Hand – Peace Operations, Organized Crime and Intelligent International Law Enforcement. International Peacekeeping, 16(1), 151–168. doi:10.1080/13533310802485633

Yes, it started with lies as an aggressive war. But yes, beyond all the catastrophy and many mistakes: Not few activities of statebuilding peacekeeping and are legally based.

I personally look on COIN (which started far too late) , on processes of reconciliation or such of civil overseeing of police. 

All in all: Legal or illegal: one will need an in-depth-analysis on different levels from the micro to the international sphere. 

_____

Some other relevant sources - though I am sure you have them: 

A More Secure World: Our Shared Responsibility, Report of the High-level Panel on Threats, Challenges and Change, UN-Dok. A/59/565, 2. 12. 2004, Abs. 201ff. 

Sabine von Schorlemer, Die Schutzverantwortung als Element des Friedens. Empfehlungen zu ihrer Operationalisierung, Stiftung Entwicklung und Frieden, Policy Paper Nr. 28, Bonn 2007.

Coen, A. (2016, March 19). The Responsibility to Protect and the Refugee Crisis: Peacekeeping Challenges and the Responsibility to Protect (R2P). Retrieved from https://sustainablesecurity.org/2016/03/09/the-responsibility-to-protect-and-the-refugee-crisis/ . 

Best from Koblenz, 

Frank Thomas

Also note that the R2P or the Schutzverantwortung was adopted and entered into force in 2005. I am not sure whether R2P applies retrospective to all events you have mentioned in your question above. Also, it should be researched, whether the interventions in Iraq and Afghanistan occurred under UN mandate, which I do not think. If there was no UN mandate, on what legal basis did the interventions happen? Was it compliant with existing provisions of international law?  

Cornelia,

here the discussion coud become very vibrant!

I agree that a UN mandate can make things clearer. 

Nonetheless, as the SC acts along the lines of hegemonial interest og the Big Five, UN resolutions are not more than  auxiliary tools within the process of analysis,

Rwanda, Libya, Syria, Sudan and other regimes provide evidence that trusting in Un can be fatal. The Nato-mission in Afghanistan is backupped by UN: (http://www.un.org/depts/german/sr/sr_01-02/sr1386.pdf) 

As to resolution1441 (IRAK) and the historical background a brilliant summary, including legal sourcces, can be found at:: https://www.lpb-bw.de/irak_konflikt.html

Best from here,

Frank   

Iraq was the best example in this case. The commitment from invasion states to rebuild the destroyed country moreover, they should follow the International Law especially after publishing the British inquiry report about 2003 Iraq War.    

Australia, Canada, the US, are all recognised as states yet all began with foreign invasions that led to murder and slavery or similar.  History is full of examples.