It has come to something when the world's most profitable technology company chooses to depict itself as the proverbial seven-stone weakling.

Yet that is precisely the pose that Microsoft struck this week when it announced a long-sought alliance with Yahoo in the online search business. The identity of the muscle-bound bully kicking sand in Microsoft's face? Google. No matter that the internet upstart will not even be a teenager for another couple of years: in the tech business, kids grow up fast.

A day later, Steve Ballmer, Microsoft chief executive, summed up his company's dilemma. “In this industry, probably the greatest source of economic value creation is choosing to be in the right businesses early enough,” he said. “And when you don't choose to be in early enough, you have a problem.”

Microsoft's anti-Google strategy has been a long time in the making. It stretches back at least five years, to when Microsoft launched its first search engine, and intensified two and a half years ago when Mr Ballmer set out in pursuit of Yahoo.

The strategy has come to fruition, though, at a moment when Google's growing bulk has started to cast a long shadow over Silicon Valley. Like most loveable kids, the company was tolerated in infancy, when its quirky traits – such as the corporate motto “don't be evil” – seemed endearing. But precociousness eventually wears thin.

Google has never been averse to making enemies in other industries, such as media or communications. Losing friends in the tech world, however, is a newer phenomenon. To judge by events this week, it is something it should start to get used to.

The first was Yahoo's defection to the Microsoft camp. Though far less dramatic than the full acquisition that Microsoft proposed last year, this is a defining moment for Silicon Valley. In California's technology centre, which has long thrived on a spirit of co-operation, Microsoft has always been the outsider – the marauding giant from the north, bent on extending its influence well beyond its dominance of the personal computer.

Local connections in the Valley are deeply personal. Sergey Brin and Larry Page, when looking to turn their postgraduate science experiment into a company they later called Google, turned for advice to two other former Stanford University students who once had a bright idea: Jerry Yang and David Filo, founders of Yahoo. That friendship – and an ingrained fear of Microsoft harboured by Eric Schmidt, Google's chief executive – drew the two companies together last year after Microsoft launched its unwelcome takeover bid. It was only the intervention of regulators in Washington that prevented Google and Yahoo from cementing their friendship with a search alliance that would have left Microsoft out in the cold.

Times, and allegiances, have changed. With Mr Yang unceremoniously pushed aside as chief executive late last year, a new pragmatism has descended on Yahoo in the shape of feisty, straight-talking Carol Bartz, his replacement. For Mr Yang, who long claimed to “bleed purple” for Yahoo, the result would once have been unthinkable – Mr Ballmer wielding an outsized purple pen on Wednesday to sign the deal that will link the two companies' internet businesses.

A second Silicon Valley friendship has also shown signs of fraying for Google this week.

Its alliance with Apple had become one of the defining relationships of the Silicon Valley that emerged this decade from the wreckage of the tech bust. Mr Schmidt has served on Apple's board since 2006 and the link was cemented when Apple's iPhone was launched two years ago with Google's applications enjoying prominent display on the device. So it was a sign of the how times have changed when Apple this week banished Google Voice – an ambitious internet telephone service – from the iPhone.