By Paul Reynolds
World affairs correspondent, BBC News website
The US has a treaty obligation to help towards Taiwan's defence
A key point to remember about US arms sales to Taiwan is that they are designed to be defensive.
An American law, the Taiwan Relations Act 1979, requires any administration "to provide Taiwan with arms of a defensive character".
This is a red line between China and the US. If Washington ever sold clearly offensive weapons to Taiwan - such as cruise or ballistic missiles - China would be seriously angered and Taiwan might be actually threatened.
The proposed sales, worth some $6.4bn (£4bn), are interesting as much for what they leave out as for what they put in.
They are made up of two refurbished mine hunters, 114 Patriot anti-missile missiles, 12 Harpoon anti-ship missiles, 60 Black Hawk helicopters and ships' communications equipment.
Taiwan also wants eight submarines and 66 of the latest F-16 fighters. Neither of these requests has been met so far.
So the US offer has, in fact, been well below what Taiwan would like, at least for now.
Crossing the line?
That should secretly rather please Beijing. In public it cannot afford to show any acquiescence, so its reaction has been strong, if along well-used tracks.
Beijing's media have accused Washington of 'arrogance'
As the latest weapons systems fall within the defensive category, Chinese anger is moderated and to an extent, well-rehearsed and within bounds.
It has suspended military talks with the US. It also did so when President George Bush launched the first part of this arms sale in 2001. Relations later recovered when the immediate furore died down.
It is also threatening those US companies involved in the sale with a ban on sales in China, and it has talked of the damage to co-operation on international and regional problems.
As part of its case that Taiwan is part of China and should therefore not be armed by the US, China has also pointed out that a joint China/US communiqué in 1982 stated that the US would "reduce gradually" its sale of arms to Taiwan, leading to a "final resolution."
However, the red line has not been crossed. The broad conclusion must be, therefore, that this row is destabilising to a certain extent but not to the ultimate extent.
The relationship, while not normal given the differences of system, is manageable.
The basic policy of each side is to avoid a full-scale confrontation over Taiwan, and that is essentially preserved.
The US-Taiwan deal was for defensive not offensive weaponry
Each defines its own "One China" policy. China maintains its right to sovereignty over the island, including an ultimate threat of force if it seeks independence. The US declares its right to defend Taiwan while accepting that it cannot be an independent state.
Add in other rows over the freedom of Google to operate in China and the ongoing US trade deficit with China - $268bn in 2008, $208bn in 2009 according to the US Census Bureau - and you have a bad patch.
But bad patches usually give way to more stable periods in this relationship and there is no reason to think otherwise this time.
In these rows, both sides make the expected and required protests - the Americans over Google's freedom, the Chinese over Taiwan.
President Barack Obama's National Security Adviser Jim Jones said this month that Washington and Beijing did things "periodically that may not make everybody completely happy".
But he also said that the United States was "bent toward a new relationship with China as a rising power in the world".
The risk is that there will be a knock-on effect elsewhere. The US is currently seeking Chinese support for more UN sanctions on Iran. Any co-operation could be at risk, although frankly, China was reluctant long before the arms sales and Google reared their heads.