You don't have to strain too hard to find parallels between news today of Hammonds' merger talks with top 60 US practice Squire Sanders & Dempsey and the tie-up between Denton Wilde Sapte and Sonnenschein Nath & Rosenthal. The size of the prospective union, which would create a global top 50 practice with revenues of well over $625m (£403m), is closely comparable to SNR Denton, which itself goes live next month.

In both cases the US suitor is much larger and more profitable than its UK partner – though all of these firms have generally lagged their peers in terms of profits. Market position is also broadly comparable, even if Dentons obviously brings much more City muscle to its union than Hammonds has ever achieved.

But the greatest point of comparison is the essentially defensive nature of the tie-ups. If you were to take a 10-year view of the UK's top 25 law firms, judged purely on the numbers, Hammonds and Dentons would be the two firms that have most struggled to deliver on their considerable promise. Indeed, it speaks volumes about the reverses that have beset Hammonds over the last decade that many now forget what a hugely potent brand the firm once was. Go back to its mid-1990s heyday and it was the then Hammond Suddards that many were betting would prove how far a regionally-bred law firm could go, not Dibb Lupton Alsop (which went on to become the DLA in DLA Piper).

The loss of that status was quick and not pretty: heavy expansion costs and a City office that struggled to gain traction strained Hammonds' finances. Soon the firm was facing an exodus of partners, overpaid drawings and plummeting profits, a situation which culminated in the firm's decision in 2005 to put in place a partnership lock-in to stabilise the ship.

While some were expecting such tactics would fail, it is to the great credit of the firm and in particular managing partner Peter Crossley, who was on the first wave of the clean-up crew, that the doubters were proved wrong. Over the last five years the firm has continued to play a tough hand extremely well, but there has been no escaping the feeling that Hammonds wasn't going to regain its former vigour without doing something large and structural. Enter Squire Sanders (which had informally discussed a tie-up with Dentons before the Sonnenschein deal).

Despite having built a large US practice and a comprehensive network across the Central and Eastern European region, Squire Sanders has a few issues of its own. Its profits per equity partner for 2009 of $795,000 (£521,000) are well ahead of Hammonds' 2009-10 figure of £364,000, but that remains well below the $1.2m (£774,000) average across the Am Law 100. The firm, which last year saw veteran chairman Thomas Stanton hand over to James Maiwurm, has explored a number of mergers over recent years without closing a significant foreign deal.

Yet if the proposed tie-up is defensive, that appears strongly in its favour. It's an irony of strategic unions that deals done in such circumstances tend to do better than mergers between firms on a clear upward slant. Mergers often flounder because two sides believe in their own superiority and refuse to integrate, promoting an insidious wistfulness for the good old days. There's nothing like a nice run of calamities and dead-ends to make one constructively minded, helpfully self-critical and focused on the future. Perhaps all law firms considering a merger should engineer a few disasters before hand to sharpen their resolve.

There is one interesting wrinkle that is worth noting with the deal: the 436-word statement the firms issued announcing the talks, aside from making the mandatory nods to'global coverage', 'shared culture' and 'ambitious aims', also makes no less than four separate references to providing value or cost-effective services. As an explicit aim it should give the combined practice a little more distinction since many law firms see going global as a means of escaping domestic price pressures.

Turning to the wider market, it seems apparent that the recent union of Hogan & Hartson and Lovells is delivering on the promise that it would kickstart a run of transatlantic moves, even if those in its wake have been forged more out of strategic necessity than the vision thing. If these are the kind on deals that are on the agenda, there could be three more done by Christmas.