"Denmark, Norway, Iceland, Belgium, Netherlands, Luxembourg, San Marino, Monaco - all occupied, despite many of their attempts to maintain neutrality. The UK was considering invading Ireland. "
Re Denmark and Norway:
http://en.wikipedia.org/wiki/Plan_R_4
This is a rather extraordinary example where the British tried to stage an invasion of Norway because Norway was a trading partner with Nazi Germany. The German reaction and invasion was quicker then expected, aided by Denmarks rather quick surrender.
I don't recall Iceland ever being invaded.
"Belgium, Netherlands, Luxembourg"
This was just as in world war 1. Germany invaded France through the lowlands to get past the maginot line
San Marino: The state was successfully neutral until both the axis and allies went through their lands. This was done first by the allies who thought San Marino had already been annexed by the Nazi forces. This is more of an example of military fuck ups then actually imperialist invasions of city-states.
I will yield you Monaco.
As for the pacific theatre, my knowledge of it is less extensive then Europe, but too many knowledge the Japanese invasion was entirely against China, European colonies and America. I can't think of any minor independent countries in the region that were invaded by Japan.
"As for Liechtenstein, they had a substantial amount of land seized from them by Poland & Czech. "
That is different, Poland and the Czech Republic took land away from Germany after world war 2, most of it being private land owned by Germans, some of that land included land that was inherited to House of Liechtenstein prior to world war 2 begining. It wasn't land owned by Liechtenstein's government, but private land owned by Liechtenstein's head of state.
Stalin and Truman when they redrew the map of Europe following world war 2 recognized the Czech and Polish rights to the land. This was one of the many cases where they botched up the new map of Europe.
Besides World War 2 is an unusual case since I am willing to bet more neutrality was violated in world war 2 then every other war in history combined.
"For example, China's neighbors, even Vietnam, look to the US for help against China. To them China is a more immediate threat than the distant United States. Ditto Russia's immediate neighbors. Many of India's neighbors look to China to balance against India (Sri Lanka, Nepal, Bangladesh, and of course Pakistan all are friendly with China). "
This is a very flawed argument. India has had a massive number of border dispute with its neighbours, including China since its independence. China and Pakistan are natural allies due to border disagreements.
South East Asia doesn't have negative relations with China, and neither does North Korea (due to again them being natural allies based on previous wars and communist cultures)