Personally I believe the charges levied by the US have been trumped up, and even if they were true the offense were minor and this amounts to overzealous prosecution. So putting aside for a second whether the charges against Assange are valid or not. This is the line of reasoning being used:

1) Assange allegedly assisted in hacking US Defense Department computers. The means that he allegedly broke US laws on virtual US soil. They are also alleging that he was a conspirator that directed crimes on US soil via his agent, Manning. If he say hacked a Russian computer and the US was prosecuting him it would be a different story.

2) The US and UK have very strong extradition treaties and will generally extradite with very few questions asked. In most cases the legal systems of the US and UK are trustworthy and compatible enough for this to be a fairly reasonable thing.

Again ignoring the specifics of this case, these general principles of law are generally good. The US, and international community have a strong interest in protecting their assets. If you want to virtually strike at a country in a targeted attack you should be prepared to face the consequences -- sitting in another country while you perform it is not a valid excuse. Imagine a more clear cut case, some Russian hacker group hacks a US bank and steals millions of dollars, that seems reasonable to prosecute right?

The US doesn't care much where you physically are when you commit a crime against the US. For instance, if you conspire to traffic drugs into the US, the US can come after you. It doesn't matter if you're physically in the US or not when you do it.

If an estranged noncitizen parent living abroad conspires to kidnap his children from their custodial parent inside the US, he can be tried for kidnapping by the US. Even if his part of the conspiracy never takes place inside the US. The alternative- just letting him off, due to where he happened to physically be at the time- would be fairly absurd. The jurisdiction he's physically in probably doesn't care, and even if they do, most of the witnesses and all of the victims are physically in the US.

The US can't kidnap him but asking the jurisdiction he is in to extradite him is totally normal. This is what extradition agreements are for, in part.

> So USA laws don't apply to him, unless he is in the USA and has committed a crime or offense of some kind

Physical presence is an odd requirement. So if I pay people to deliver bombs to people in the USA, but I'm not in the USA, I haven't committed a crime?

Well... Depends. It might not be a crime in the country "X" where you are. It might be a crime in the USA, but country "X" won't extradict you because it doesn't consider it to be a crime. Even if you are a citzen from country "B", where B and USA consider it a crime.

International crimes depend on a lot of factors.

Yes, but not in the USA. You did not bomb - you paid for it, in another country that should judge that.

Army size always correlates to more diplomatic power. You also have US military bases everywhere in Europe. Not the reverse. Europe relies heavily on US troops for their defense. So if the US perceives Assange as the threat, the rest of EU better does as well! It's that simple.