Mark Mardell: Vietnam is the Ukraine of the Pacific

There have been at least three incidents reported between Vietnam and China in disputed waters, including this one in which a Chinese ship (left) used a water cannon on a Vietnamese ship

There's a view that President Barack Obama has had some limited success in deterring Chinese aggression in the South China seas.

Very limited. They waited until he was gone.

Days after Mr Obama's Asian tour ended, the Chinese squared up to Vietnam and planted an oil rig in disputed waters. They sent in 80 ships, including seven military ones, with one firing water cannons at the Vietnamese.

The Chinese accused Vietnam of violating international law.

The state department has condemned this as "provocative and unhelpful" to security in the region. adding, "We are strongly concerned about dangerous conduct and intimidation by vessels in the disputed area".

'Concerned'

Under Obama the US may huff and puff but not even threaten to do much more

The state department has also ticked off the Chinese for apparently hunting sea turtles in Philippines waters.

"Given the United States works with the international community to combat wildlife trafficking, we are concerned that the vessels appear to be engaged in direct harvest of endangered species of sea turtles," a state department spokeswoman said.

That's not quite as stern, and oil leads to war more often than rare reptiles, but here's where the Ukraine comparison comes in.

The US has a military relationship with Philippines and Japan.

There are severe doubts in the region how much muscle the US would flex if push came to shove.

But Mr Obama has indicated he's standing behind them. Just as he's said he stands four square behind the Baltic States because they are members of Nato. Whereas Ukraine is not.

Similarly, the US has no military agreement with Vietnam.

Relations are warmer with the former enemy than in the past but that falls very far short of any formal or informal promise of protection.

That's why diplomats here in Washington think Vietnam could be the Pacific's Ukraine - the US may huff and puff but not even threaten to do much more.

China is watching events in the former Soviet Republic very closely.