James Goldrick's thoughtful response to my last post raises lots of important issues. Let me touch on two of them.

First, James says that my argument for sea denial over sea control focuses too much on high-intensity conflicts and especially power projection in such conflicts. 

James says we need to be able to use the sea for trade as well as power projection and of course I agree. He goes on to say we therefore need maritime forces that can maintain sea control to allow that trade to continue. That is a different claim. Whether it is true or not, or the extent to which it is true depends on what kind of threats we believe our seaborne trade must be protected against and what that means we need.

One can argue we need to be able to protect trade against low-level threats such as piracy and we therefore need some small warships capable of dealing with such threats; something like the present ANZAC ships, in fact. On that I'm sure James and I agree.

The question is whether we need to be able to protect our trade against the kind of bigger threat that could only be posed by a country with modern maritime forces. This matters to our force structure because ANZAC ships (or rather, as James reminds us, a complex system of systems in which ANZAC ships are the most capable surface ship element) would not be able to achieve sea control sufficient to defend any serious fraction of Australia against the forces of a capable maritime power. If we need to do this, we need a much bigger and more expensive navy.

So two questions arise. First, how seriously do we need to take such high-end threats to our trade? They used to be a major issue in bygone times, and they loom large in classic conceptions of naval strategy which remain very influential today. But that was based on the experiences of an era of 'national' trade, when it made strategic sense for European powers to attack one another's colonial traffic.

It is now a very long time since trade worked that way and the globalisation of the past few decades has pushed us further from that model still. This is a big topic, but let me just baldly assert that economic interdependence makes threats to trade from substantial maritime powers extremely unlikely except in a general war in which all trade would be massively dislocated. Short of such a war, for example, who would gain from attacking Australia's overseas trade, or anyone else's, and who would lose?

If I'm wrong, how could we best respond to such attacks? No conceivable Australian force structure would allow us to protect even a tiny fraction of Australia's trade from attack by a substantial naval power. But any adversary would be in the same position. His trade would be just as vulnerable to our attacks as we would be to his. So our best defence is attack. That means we should invest in sea denial to protect our trade, not sea control.

Second, and very briefly, James says we will need sea control to mount and sustain a sea denial campaign. He may well be right. I'm not sure that sea denial against a major adversary is achievable for Australia; support and replenishment for our forces is one of the key question marks. If sea control is in fact unachievable for Australia against a major Asian power operating in our maritime approaches, then Australia may prove to be undefendable against such an adversary.

 I'm not arguing that sea control would not be nice to have, but before we invest in it we need to know that the aim is achievable. If it's not, we need to ask what can be achieved without it. My point about sea denial is that it's better than nothing and I suspect that is the alternative.

Finally, the underlying point about this debate goes to the way we think about designing our forces. We cannot make robust decisions about what kind of ADF we need unless we get the clearest possible idea of what exactly it needs to be able to do, and what exactly is the best, most cost-effective, way to do it. I think it is important that Navy does much more to explain in specific operational terms what the capabilities it wants to acquire will be able to do, where and against whom, and then explain how that serves Australia's strategic objectives cost-effectively. 

 Photo Lauren Black/ADF.