Forced Democracy?

What rights does an external government have to influence whether or not another government is a democracy? This question was brought to mind today in seeing the overthrow of the government in Thailand, the violent protests in Hungary, and the looming conflict in Mexico.
As an illustration, to me it seems that the USA believes they are fully entitled to interfere in the politics of whichever country they please, essentially practicing the modern form of imperialism. Take for example the elections in Ukraine in 2004. When allegations of fraud surfaced, the US and other western countries were very, very involved in ensuring that the elections were examined, and a new election done. This almost certainly had to do with the fact that the pro-western candidate (Yushchenko) lost to the pro-russian candidate (Yanukovych) in the runoff election (Note that Yushchenko won the preliminary election but in accordance with Ukrainian law, because no candidate had 50% of the vote, a runoff election was held). By no means do I intend to say that a just result was not achieved as I consider that the original runoff election was indeed fraudulent and that Yushchenko was the rightful winner. However, I do intend to compare this with the involvement of the western nations, or rather lack of involvement, in the 2006 elections in Mexico. Despite an almost equivalent situation, there was negligible political involvement of western powers in ensuring that the election held was indeed fair. The difference: the conservative candidate Felipe Calderón defeated the central-leftist Lopez Obrador.
What it seems to me is rather than influencing other governments to become democracies, powerful countries use this as a guise to encourage the rise of foreign governments that are economically beneficial for them.
What right then do governments have to interfere in the political structure of another country? Is there a just reason for invading a country to change its political structure, and if so how can one make that judgment? Furthermore, how can the country that would thus do the changing be monitored and kept in check? Can change caused by outside forces be sustainable in the long term, or must the change come from within the country?
Sorry, this one ended off with more questions than answers, but I have a feeling it's to be continued... Comments!