There are no easy options for South Korea - or the United States - after an international investigation group reported that a North Korean torpedo sunk the South Korean naval vessel Cheonan in March.
Military action is out - it carries with it the risk that the North would retaliate and might even launch action of its own.
North Korea has a million-strong army, 18,000 artillery pieces and possibly useable nuclear weapons.
Sanctions have limited impact on the North. It is already ignoring UN sanctions restricting trade in military equipment and luxury goods.
It manages to keep going with Chinese help and that is unlikely to be withdrawn.
It might be that the South is left to make a lot of diplomatic noise, with American echoes.
In the end it might have to accept that the lesser of two evils is to make its factual case, protest, gain international support, limit its own dealings with North Korea and then continue to build up its forces for the future.
The China dimension
The sinking of the Cheonan is not the first incident along the line of the disputed maritime boundary off the west coast.
None has led to outright war and there is therefore reason to think that this time as well, the South will have to live with the loss, unless it wants to risk a major conflict.
The investigation's findings have thrown the issue into the lap of US Secretary of State Hillary Clinton, who happens to be arriving in Beijing this weekend with a huge American delegation to discuss the wider relationship between the US and China.
It is possible that the US, pressed by South Korea, will favour a Security Council meeting and, if such a meeting is to have any traction, China's support will be needed.
And beyond that, if new sanctions are to be imposed, China will have to support them as well, being a veto-holding member of the Council.
Yet China usually prefers to deal with North Korea on a more discreet basis (it recently welcomed North Korea's leader Kim Jong-il to Beijing) and was slow even to express sympathy for the loss of South Korean sailors.
It only comes out against the North if Pyongyang does something with wide international ramifications, notably its nuclear testing.
The US might, at this moment, be more concerned to keep Chinese support for sanctions against Iran - agreed by the major Security Council powers this week - than to use its diplomatic credit up in seeking further sanctions on North Korea.
The prospects for settling the disputed maritime border are about as distant as the prospects for getting North Korea to give up its nuclear weapons capability. Talks about the latter are currently in suspension.
North and South Korea did nearly come to a maritime modus vivendi in 2007 in talks involving the then South Korean President Roh Moo-hyun.
The idea was to concentrate on fishing - and fishing is a major North Korean concern in the disputed waters as its fishermen seek the south-migrating blue-crab from June to September.
But it came to nothing and the North clearly remains ready and willing to defend its interests there, with the South determined to uphold its position as well.
Some international experts think that what is known as the Northern Limit Line, drawn by the UN after the Korean war, should be moved in North Korea's favour, since the old international three-mile maritime limit has made way for 12.
But given the overall state of relations between North and South, there are no realistic hopes of an agreement.
There may well be further incidents ahead.