By David Benoit And Liz Hoffman 

On a Sunday evening in early October, Valeant Pharmaceuticals International Inc. Chief Executive Michael Pearson met the activist investor William Ackman at New Jersey's Teterboro Airport. On the agenda for the globe-trotting duo: whether to raise their $53 billion bid for Allergan Inc.

Their hostile pursuit of the Botox maker was in its sixth month and Allergan appeared to be slipping out of their hands. The pharmaceutical company was said to be exploring a large purchase of its own or selling itself to another suitor.

Mr. Ackman argued that a decisive increase in the offer for Allergan would create a surge of momentum, drawing more investors who supported the deal into Allergan shares and boosting Valeant's stock, too. Mr. Pearson, who already had increased Valeant's bid despite pledging not to bid against himself, said it was too soon to offer more. He didn't want to use up their fire power only to get topped by a rival.

Their debate continued until this Monday, when their fears were realized: Allergan announced it would be acquired by Actavis PLC, a rival drug maker, for $66 billion. Valeant bowed out, confirming Wednesday it wouldn't compete with the $219-a-share offer.

It had been an unorthodox--and widely watched--alliance, teaming an activist investor with a corporate buyer to try to land one of the year's biggest deals. Their travails show that the setup created tripwires that don't typically surface in takeover attempts.

The two men didn't always see eye to eye, people familiar with the matter say. The arrangement didn't deliver all that each side hoped it would. The tie-up brought heavy legal scrutiny. And, ultimately, it didn't land the deal.

Still, even in defeat, the two sides can look forward to a tidy profit. Valeant and Pershing Square Capital Management LP, Mr. Ackman's firm, stand to make a combined $2.6 billion on paper from the Actavis deal price. The people say the men remained on good terms throughout.

"It was a great partnership with Valeant," Mr. Ackman said. "Our respect for them only increased over the term of the transaction."

This account of their pursuit of Allergan is based on interviews with people familiar with Valeant and Pershing Square, as well as court documents and filings.

The partnership started in February when the two camps hatched a plan to take over Allergan. Pershing Square would build a 10% stake to pave the way for a potentially hostile fight, led by Valeant.

"It was like a nuclear launch device; both guys have a gun, both guys have a button," Mr. Ackman later said in connection with a lawsuit filed by Allergan about Pershing Square's setup with Valeant.

The first cracks in the partnership emerged early. Valeant hoped one benefit of teaming with Mr. Ackman would be his reputation as a stock-picker. The company had been targeted by bearish investors who argued its acquisition-driven business model wasn't viable in the long term.

Mr. Pearson hoped the imprimatur of Mr. Ackman, who had spent months studying the Canadian drug maker, would discredit short sellers and buoy Valeant shares.

On April 22, the day the pair made their offer official, Mr. Pearson prodded Mr. Ackman to be more explicit in his support of Valeant. In an email with the subject line "You've been busy with the press," Mr. Pearson said: "Maybe you should also emphasize quality of our company," according to court papers.

Mr. Ackman made the effort, speaking extensively that day about Valeant at a public presentation on the bid, praising Mr. Pearson and Valeant's performance.

But questions kept swirling about Valeant's business model, which Allergan also hammered.

In June, Mr. Pearson told The Wall Street Journal he was surprised that Mr. Ackman's blessing hadn't quieted naysayers.

"We thought that validation would be, maybe, more helpful than it's been, " he said. Some in the Pershing Square camp bristled at the remark.

Valeant didn't help its cause in July when it released results that disappointed the market. Pershing Square felt Valeant did a bad job guiding Wall Street.

Meanwhile, the Valeant side fretted as Mr. Ackman's reputation suffered in another investment: his campaign against nutritional company Herbalife Ltd.

In July, Mr. Ackman delivered a presentation he promised would be a "death blow" to Herbalife. Instead, the stock recorded its biggest one-day percentage gain ever. That day, Valeant shares slipped.

Valeant advisers worried Mr. Ackman's public stumble would weaken their joint campaign. Yet hours later, when Messrs. Ackman and Pearson spoke with Allergan investors in Toronto, the two men appeared friendlier than ever.

Such investor meetings were integral to the duo's strategy. At one, in May, some shareholders that bet on events like mergers suggested the team should launch a tender offer, seeking to buy Allergan shares from investors despite the board's resistance to the deal.

Three weeks later, the duo followed that advice, but it proved far more controversial than they expected.

Regulations prohibit an investor like Mr. Ackman who knows about a coming tender offer from buying stock in the target company if there have been "substantial steps" taken toward the offer. Pershing Square and Valeant said they hadn't taken any steps toward a tender offer back in April, so believed they were in the legal right.

But the tender offer move was the opening Allergan had been waiting for. The company accused the duo of insider trading in a federal lawsuit.

A judge earlier this month found there were "serious questions" about whether Valeant and Pershing Square had broken the law and ordered them to make additional disclosures, but let them proceed with the bid.

Mr. Ackman came to regret launching a tender offer. It had been meant to be a largely symbolic effort to assure shareholders, but turned into a legal and public-relations headache.

The day after the judge's decision to let the Valeant bid proceed, Allergan entered into a confidentiality agreement with Actavis. By early October, as news leaked that Allergan was weighing other options, the men had their meeting at Teterboro to strategize on whether to bump the price.

Mr. Ackman wanted to move quickly. The Valeant side wasn't convinced. Valeant had earnings in two weeks and executives expected to surprise investors with their strength. The market did cheer the results, but Mr. Pearson had begun to believe the noise was keeping his company's stock depressed. That made it more expensive to use in bumping the price.

In the end, the battle ended like most bidding wars do, with the prize going to the highest bidder.

Write to David Benoit at david.benoit@wsj.com and Liz Hoffman at liz.hoffman@wsj.com

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