The situation has grown beyond the title "US/Japan Alliance" so I'm making a new thread. So things like joint-exercises and such will go here.
The number one cause for the formation of this, what I would not be too hesitant in calling Asia-Pacific allies, is China's activities in the South China Sea. There are many separate issues that exist between China and any of the other countries in the Asia-Pacific such as 1) Japan and the East China Sea, 2) South Korea and situation with North Korea, 3) US and cyber theft, etc, 4) Taiwan and independence status, and so on. But it is the South China Sea that puts China against the interest of many countries single handedly, creating a common ground for all these countries to form security relations.
At the moment, the most important members of these allies are, perhaps in the giving order, are the US, Japan, the Philippines, Vietnam, Australia, and Taiwan. I would name the US and Japan first because that is where the power is coming from. Without them, China can do as it pleases. But Vietnam and the Philippines have the important position of being at the front line. Australia is additional help via its own interest in keeping China in checked to not let China because too powerful in waters south of Indonesia. And Taiwan is involved because it has one island, although perhaps the largest of the naturally occurring islands in the South China Sea, with a 1,200 meter long runway. And Taiwan has the Pratas islands further up north that doesn't get much news, although they are a "natural park" so probably no military installations on those.
What makes this not like the Cold War between the US and the SU in the second half of the 20th century is that all these member countries have have large economic relations with China whereas I'm pretty sure the economies of the West and the SU were very separate, certainly much less than the economic connections between China and these Asia-Pacific allies. These economic relations evolved to a large volume about 10-15 years more or less before the security issue in the South China Sea became severe enough to cause the coming together of these allies. How much longer can economic relations last? How long can economics prevent escalation to war. I'm reminded of the economic relation between the US and Imperial Japan. Japan invaded Shanghai and Nanking in 1937, fought major battles against the Chinese though to 1938 and a long stalemate war ensured afterwards. The US didn't start really pressing Imperial Japan economically until 1941 with the oil embargo in July 1941 IIRC. In short, I do not think that the current economic situation guarantees the prevention of an actual shooting war to occur. Especially if CCP China might see that a small and quick shooting war might serve to actually benefit CCP's hold of dominant power on China.
One thing to note is that with the Imperial Japan and US example above, what probably caused the US to take economic action against Imperial Japan was the possible risk of outside enemy powers to exceed US power, namely a Europe being dominated by Nazi Germany and an Asia dominated by Imperial Japan. It is pretty difficult to imagine China exceeding the power of these forming Asia-Pacific alliance, so it seems likely that China won't be tempted to actually start a war. But like in the Imperial Japan and US example, there were other factors in the world, namely Nazi Germany. In today's world, there's Russia, Iran, and North Korea. So well, in short, we'll see.
One of three other things worth mentioning are potential future members to the Asia-Pacific allies. Those countries are Indonesia and Malaysia. They have maintained a neutral position towards China on one hand and the Philippines/Vietnam/the US/Japan on the other. But recently, I would say they have slightly slanted their position towards the latter but time will tell if they go further in that direction.
The second point are somewhat associated counties with the Asia-Pacific allies. These are countries that haven't raised the South China Sea as a primary concern, for a number of reasons respective to their cases. These are India, South Korea, and maybe New Zealand and Singapore. India will use the tactic of throwing its weight into the SCS as a means of countering China's moves to grow its power in the Indian Ocean. South Korea is by large tied up with North Korea so are in a delicate diplomatic situation with China and probably won't want to get involved in the SCS because of that like how Japan is able to get involved. Although they still seem to no mind selling weapons to say, the Philippines, weapons that obviously benefit the Philippines in is struggle against China in the SCS. Also, South Korea and the US share their own security alliance. New Zealand seems to tag along where Australia goes. Singapore sometimes hosts US military assets but that doesn't necessarily put Singapore diplomatically opposed to China in the SCS.
The final 3rd post are counties more aligned with China in proximity to the SCS. These are Laos and Cambodia. These two countries seem to sometimes disrupt ASEAN processes on the behalf of China.
The best possible outcome if that while China is militarily kept in check by these allies, that China will transform from within to become a more transparent and open society and that China makes changes their 9 dash line stance. So it'll be a long time until that happens. On this long road, hopefully no fighting actually breaks out.
So to start off, the 2016 Balikatan exercises between the Philippines and the US just got underway which will have 5,000 US troops, nearly 4,000 Philippine soldiers, and 80 from Australia.