Reading about multi-billion dollar M&A activity gets old after a while. What about the ones that didn’t get across the line?

Quite rightly, people (and some lawyers too) get excited about the prospect of a multi-billion dollar takeover or merger.

The whispers between well-heeled corporate types are enough to keep the business pages filled, and the various ins and outs contain enough detail to keep business nerds in rapture.

That’s because the bigger the deals, the bigger the ripples of the deal.

Truly huge mergers can cut across various jurisdictions and lead to several governments having their say on the deal.

They can also lead to rampant speculation about the shape of the new entity and the fate of projects already underway.

After all, very rarely do acquisition or merger proponents float the idea of a massive deal to keep things the same.

In much the same way it’s fun to ponder ‘sliding doors’ moments in history, it’s worth going through some of the failed mega mergers to see what may have been.

1. Pfizer deal for Allergan Plc falls through ($160 billion)

The year was 2015, and the conversation about international tax dodging was just starting to pick up steam — and the biggest failed deal in history was the result.

Pharmaceutical giant Pfizer wanted to do a deal with Ireland-domiciled Allergan Plc, and you can probably guess where this is going.

While Pfizer at the time was talking up synergies and what have you, the real aim of the transaction was to take advantage of the lower corporate tax rate in Ireland.

It had become a trend at that point to try and domicile businesses in lower tax jurisdictions, and while plenty of companies managed to head off to the Emerald Isle to take advantage.

But in 2015 the US Department of Treasury started to crack down on the practice, passing ‘Tax Inversion’ legislation which under the terms of the deal qualified as a “Adverse Tax Law Change”, leading to the mega-deal falling by the wayside.

Ironically, Allergan was largely the result of tax inversion deals — with US-domiciled Actavis acquiring Ireland-domiciled Warner Chilcott in 2013 — with the new entity then acquiring botox-maker Allergan, leading to the new name.

And they would have gotten away with it too if it weren’t for those pesky regulators.

2. BHP and Rio walk away ($148 billion)

A little closer to home, but it was only a decade ago that a crazy plan was floated to merged BHP and Rio Tinto, leading to a super-giant mega commodities behemoth.

But before the deal could truly test the mettle of regulators and nervous customers worried about the effect of the deal on prices, commodity prices tanked.

While regulators had suggested that the combined entity would need to sell off assets for the deal to get over the line, BHP was willing to entertain that idea.

READ: The humble beginnings of 10 massive companies

Adding to the downside for BHP was Rio’s $38 billion in debt, compared to BHP’s $6 billion.

Again, this would have been surmountable if commodity prices hadn’t tanked — but with commodity prices plunging and the broader market shaky it was deemed too risky to proceed.

BHP and Rio would dance again, but the 2008 deal was the main game.

3. Kraft Heinz can’t convince Unilever ($143 billion)

Last year Kraft Heinz tried to convince Unliever’s shareholders to become part of a truly scary FMCG (fast moving consumer goods) giant — but the deal didn’t go ahead.

While whispers may have been mounting behind closed doors, the remarkable thing about this failed takeover bid is that it was dropped just two days after it was first floated publicly.

Ironically, it appears a leak to the Financial Times forced Kraft Heinz and Unilever to reveal that a deal was in the works — which spooked regulators and investors.

Unilever executives claimed the deal undervalued the company, there were noises about potential job cuts, and there were the usual rumblings about regulator discontent.

4. MCI Worldcom fails to acquire Sprint ($129 billion)

A deal between two major long distance telecommunications providers in 1999…failed to go the distance, when regulators started to get involved.

The number two and three long-distance providers in the US were aiming to really tackle AT&T head-on — but the deal was scuppered due to anti-competition concerns.

Regulators wanted more balance in a market where Worldcom, Sprint, and AT&T controlled a whopping 80 percent of the long-distance market.

The US Department of Justice went as far as to file a lawsuit against the deal.

There was also the small matter of the European Commission prohibiting the deal as well — meaning in the end the pair put the merger in the ‘too hard’ basket.

5. Pfizer walks from Astra Zenaca deal ($123 billion)

While Pfizer’s deal for Allergan failed due to tax minimisation/avoidance issues, a mega-deal a year earlier fell through due to resistance from the UK government.

It initially floated an offer worth $106 billion, but later tried to sweeten the pot to $123 billion.

There were a couple of issues at play which contributed to this deal falling over, including pressure in the UK to scupper the deal on fears a Pfizer takeover would lead to job losses there.

Under what was undoubtedly some political pressure, Astra Zenaca said the deal ‘undervalued’ the company and decline the merger — and Pfizer had no intention of following up with a hostile bid.

6. The Broadcom/Qualcomm merger saga ($121 billion)

Mergers usually have rounds of offers and counter-offers, and this one is no different — with the possible exception of the deal being effectively scuppered by none other than Donald Trump.

The takeover dance evolved over two years, with Singapore-based chip maker Broadcom initially lobbing in an offer with $103 billion, which was politely declined.

However, Broadcom came back with an offer worth $121 billion a short while later (revised downwards to $117 billion) — but the US-based Qualcomm board also declined that offer, even going as far to say that customers would lose faith if it accepted the offer.

Crucially, the offer went hostile — prompting regulators to take a look at the deal.

But the real kicker came when Donald Trump opened his mouth to voice his concerns on the deal.

He said that there was “credible evidence” that the potential acquisition may “impair the national security of the United States” if it was able to get its hands on Qualcomm.

What made Qualcomm an attractive target was that it was it was a leader in the field of developing 5G chips — followed by Broadcom and Huawei.

Broadcom had by this stage developed a reputation for selling off assets, prompting fears that the lead Qualcomm had in the development of 5G chips would be eroded — crucially giving Huawei a chance to catch up.

So, Trump said no.