GIVEN their preference deal, few observers of politics can have been surprised at the Gillard-Greens "alliance".

Agreement is one thing however, whether it can it serve as a foundation for stable government is a completely different matter.

On issue after issue, Labor is, as it were, from Venus and the Greens are from Mars. And in case it has escaped the independents' attention, the divergence in Greens and Labor ideas on foreign and defence policy is as great as in any area.

Foreign policy was little debated during the election campaign, but this will change if the Labor-Greens alliance is to be an element of a new Gillard government.

While the Coalition will be determined to engage the alliance on foreign affairs, whether it is in or outside government, the internal Labor-Greens debate will be no less intense or less interesting a spectacle.

Indeed if both sides remain true to declared policy principles, we could be in for considerable turmoil as Labor and Greens slug out the basis of a viable policy.

This would introduce a high level of instability into the conduct of Australia's foreign policy and compromise our ability to act as a constructive actor in international affairs.

If the Greens were to prevail in some of these debates, we could be confident that some of our most important international relations would be at risk.

The problems would start with the personalities. If Stephen Smith were pushed aside and Kevin Rudd installed as foreign minister (the senior portfolio Julia Gillard has promised) two things would happen.

First, Rudd would resume his role as the chief architect and engineer of Australia's foreign relations. Second, we would have a clash of world views between two key actors at the apex of government.

The elements of realism in the deeper recesses of Rudd's foreign policy character would collide with the naive idealism that seems to animate Bob Brown's global outlook. Perhaps they could deal, but Rudd's refusal to meet Brown at critical stages of his doomed prime ministership gives some indication of the capacity for harmonious communication that already exists between them.

Assuming some kind of workable personal relationship could be achieved, differences would surely quickly emerge over any one of numerous policy matters where Labor and the Greens are hemispheres apart.

The differences over climate change are already well known, but consider the ANZUS alliance. Greens policy on this matter states that they will "end the ANZUS treaty unless Australia's membership can be revised in a manner which is consistent with Australia's international and human rights obligations". It is an entirely frivolous proposition that the alliance compromises Australia's human rights obligations in any way, but leaving that small matter to one side, the reality is that support for the alliance is built into Rudd's foreign policy DNA as it is apparently in that of Gillard.

This is a core issue of policy on which the Greens and Labor have a profound and apparently principled difference of view.

Similarly, Australia's commitment to Afghanistan. The Greens demand an immediate withdrawal of our forces from an international coalition of forces, which has a mandate from an international organisation -the UN - in which the Greens have a deep belief.

While Labor's policy towards Afghanistan has at times lacked conviction, it has nevertheless sustained (and indeed increased) the force commitment during its period of office and it intends to continue the policy.

Here there seems to be a policy wreck in waiting as a public brawl over Afghanistan looms on the horizon. This however, will do little to advance Australian interests there.

And finally on Afghanistan, if human rights are of concern to the Greens, someone from inside their cosy coven needs to explain how a withdrawal of Australian forces is going to protect the human rights of all those Afghanis (Hazaras and women among them) who will almost certainly be subject to greater abuses, should the Taliban resume control of the country.

Of course human rights issues will also confront the alliance when it comes to China. Rudd (along with the Coalition) rightly sees this as an important issue in the bilateral relationship. Sensibly, however, Labor (and again the Coalition) does not believe that Australia-China relations should be held hostage to China's record on human rights. In contrast, this seems to be all the Greens ever care about in our relations with China.

There are many other examples of the incompatibilities between Labor and Green foreign policies, but perhaps one further instance is instructive. Greens policy supports the abolition of the International Monetary Fund, World Bank and World Trade Organisation failing the absence of radical reform. Although hardly perfect, given the importance of these three organisations to the running of the international financial and trade system and the benefits that Australia derives from them, it is hard to think of a more zany idea for international reform.

By way of contrast, Labor's foreign policy convictions have always been multilateralist, as evidenced by Rudd's determined efforts to help remake the Group of 20 in the wake of the global financial crisis.

In the end there is a profoundly troubling problem with the Greens and foreign policy: more is owed to ideological conviction and idealistic purity than to the rational pursuit of the national interest. Their policies would only ensure that Australia's international voice is marginalised and some of our most important international relationships sacrificed. This is the risk to which Australia is now exposed. With the formation of the alliance, Greens ideology is about to become a powerful and malign influence on Labor foreign policy.

The alliance may serve Gillard's desperate efforts to retain government, but in the area of foreign policy it is difficult to see how it can be anything but fractious and, more importantly, how it can be in the national interests.

Russell Trood is a Liberal senator from Queensland and deputy chairman of the Senate foreign affairs, defence and trade legislation committee.