One word dominates so far the acres of comment on the government's decision to nationalise Northern Rock. That word? Inevitable. Taking the ailing mortgage bank into public ownership had become, even in the eyes of newspapers like The Mail, The Telegraph and The Sun, unavoidable.

Indeed, Gordon Brown and Alistair Darling shouldn't have dithered so much about the political fall-out. They should have nationalised the Rock months ago, say many. And now that they have, look what's happened.

No-one has bothered to besiege a Rock branch demanding their savings back. Why, the Footsie has soared by 159 points, with even other battered bank stocks showing some tentative signs of recovery.

The Tories, who have decided to oppose nationalisation, look to be the main casualties, short-term. Shadow Chancellor George Osborne did his best to paint Darling as somewhere far to the left of Michael Foot, taking "sweeping, draconian powers" to nationalise any high street bank in Britain that steps out of line.

But even Osborne, at his most opportunistic, must know how ludicrous that charge is. After all, the last two financial institutions to be acquired by the state in Britain - Johnson Matthey in 1984 and National Home Loans in the early 1990s - were rescued when the Tories were in power.

And, as the LibDem treasury spokesman Vince Cable has pointed out, what is the current Osborne solution - a Bank of England-led reconstruction of Northern Rock - but nationalisation by any other name?

So, Northern Rock will be nationalised for what its new executive chairman Ron Sandler says is likely to be "a period of some years". And that's where another rich seam of controversy immediately opens up.

Some of those arguing that nationalisation was inevitable from the start do not share the government's faith that Northern Rock can be restored to health and sold back to the private sector, having paid off the billions it owes the taxpayer.

They want it nationalised so that it can be closed down now, as clinically as possible. With all the proceeds repaying all that government lending that has kept it afloat, not compensating Rock shareholders for an investment risk gone spectacularly wrong.

That is self-evidently not the Brown/Darling stance. Sandler, the man chosen to put the Rock back on its feet, promised yesterday that, within the state aid constraints Brussels will now impose on how a nationalised Northern Rock pursues business as usual, the bank will continue to "compete vigorously" for both savers' deposits and its share of mortgage lending.

But that's a distortion of trade, wail the critics. Who wouldn't put their savings in a state-owned bank offering top-line interest rates, underpinned by a government guarantee? If you are going to push out the boat on how big a mortgage you can afford, why not borrow from a bank ultimately answerable to politicians who don't like to see anyone thrown out of their home when repayments prove too much?

They have a point. However, until we see the detail of the Rock's new business plan and what limits the EU competition authorities seek to impose on Sandler's promised vigour, how can we judge between nationalisation as a way of giving Northern Rock a second chance in the private sector and nationalisation as the best way of putting an errant bank out of its misery?

How radically will the Sandler team slim down the bank? How soon will the financial markets and demand for housing return to what can credibly be described as normal? Are product positioning that carries no taint of government underwriting and renewed market vigour compatible?

Until we get some answers to questions like these, who can say whether a state-owned Northern Rock can be repaired and refloated back into the marketplace?

David Cameron's Tory Party could have sided with the hardliners and advocated explicitly a state-sponsored liquidation of the Rock's business. Instead, they have opted for that weasel word "reconstruction". Their present tactics risk alienating everyone.

But that doesn't leave the government in the clear. Its take on why this temporary nationalisation is necessary still faces many formidable hurdles. Current Rock shareholders will get a pittance for their shares, even when set by an independent valuer. Protracted litigation seems certain.

But, in truth, that's all they deserve. Their old Rock board drove this business over the edge, with a business model which took no account of the possibility that lending on the wholesale money markets might one day dry up.

The Rock's mortgage book may still be worth upwards of 400p a share. But without £55bn of Bank of England lending and depositor guarantees this business would be bust and its shares worthless.

The bigger challenge by far for a nationalised Northern Rock is whether it can compete successfully against shareholder and mutually-owned rivals without those others crying foul.

It's been done before. That arch lefty Ronald Reagan sanctioned the nationalisation of Continental Illinois in 1984, when it became insolvent. It stayed 80%-owned by the federal government for a decade, until Bill Clinton's first administration sold it, at a profit, to Bank of America. But there's no guarantee that trick can be repeated with the Rock.