If they were unilaterally acting on their own, this could be a violation in the UN charter. As the basic idea is going against the principals of everyone working together to diplomatically resolve these issues. In the case of Iraq, the U.S. withdrew their support of the UN Resolution and led their own coalition of the willing conducting the invasion. This is indirect violation of Article 1 of the UN Charter (which prohibits nations from engaging in the use of physical force against another state). (Slomanson 1-44) Yet, when you look at these ideas a little further, it is clear that Iraq was showing how it was hostile towards U.S. And UN-based forces. Evidence of this can be seen with Saddam Hussein sending 6 thousand troops into the Iraqi dessert in response to war games that were being conducted between the U.S. And Kuwait. After this incident, Iraq had also backed down when they were faced with the possibility of military invasion such as 1998. This is important, because it is showing how Iraq considered this to be a violation of their sovereignty. As they felt that the large military presence was a direct threat against the country itself. While the military exercises, was an indication that Iraq could face a second invasion if they do not change their behavior (from the view point of the United States). (Slomanson 285 -- 349)

The reason why, is because the provisions on the use of force are showing how Iraq's claims of protecting its sovereignty were overblown. This basic approach has allowed the UN Security Council to use force in order to deal with issues. Where, Iraq had engaged in aggressive actions...

The fact that they are refusing to comply with 16 different UN resolutions is a sign that they are knowingly violating these basic provisions. Moreover, Iraq had supported terrorist related activities that were directed against Israel and the U.S. This sparked fears that they were working in conjunction with Al Qaeda. These different elements are important, because they are showing that the U.S. had justifications for invading Iraq. Yet, they did not have the votes in the Security Council to authorize these kinds of actions. (Slomanson 461 -- 550)
The implications of the Iraq invasion are that they have established a dangerous precedent for international law. As, powerful countries (i.e. The United States) can engage in actions that are in its own self-interest. Yet, they are also a violation of international law. This sets the stage for some of the larger states to follow a similar approach by using the international community to achieve their objectives. Then, when they cannot find support in these areas, is the point that they will use their own approach. Once this occurs, it can cause international law to become ineffective in dealing with these issues.

Bibliography

O'Connell, Bruce. Counter Insurgency in Iraq. Santa Monica, Rand, 2008. Print.

Slomanson, William. Fundamental Perspectives on International Law. Boston: Wadsworth, 2010. Print.

Tirman, John. Overcoming the Bush Legacy. Washington DC: Potomac Books, 2010. Print.

MLA Format. http://owl.english.purdue.edu/owl/resource/747/01/