Almost the exact reverse is actually true, in that the relationship is famed for sabotaging an alliance instead of forging one. Some background for you:

In 1173 an unforeseen diplomatic opportunity presented itself to the Kingdom of Jerusalem; namely that of an alliance with the mercurial Assassins. A fanatical Shia sect which considered its leaders to variously be reincarnations of deities or harbingers of Armageddon, the Assassins had carved a territory for themselves high in the Syrian mountains of the county of Tripoli, most notably at Masyaf. From this lofty sanctuary the sect struck at its enemies with assassinations, to decapitate chains of command and inspire fear out of all proportion with their strength in numbers. The huge regular armies of horse and foot, which Muslim and Christian alike yearned to swell in size at any cost, were found to offer precious little protection in the face of determined lone killers purposely placed in positions of trust. Over time tales of the sect's deeds would become simply preposterous inventions, but each elaboration merely enhanced their myth. Such a formidable reputation, combined with an aptitude for the asymmetrical application of violence, made the Assassins dangerous enemies indeed.

William of Tyre writes that one member of the sect named Abdallah made the journey down from their mountainous stronghold to seek an audience with king Amalric of Jerusalem, bearing not a blade but a bargain. The emissary represented the Assassin's leader, known as the Old Man of the Mountain, who at that time was the intelligent Rashid al-Din Sinan. An alliance was proposed, with even the prospect of the sect's conversion to Christianity suggested to sweeten the deal. Such claims are hard to take seriously but certainly aroused interest, just as intended. The Assassins harboured a hatred of Sunnis which appeared to make some degree of symbiosis with Jerusalem mutually beneficial; the king for his part was receptive to these overtures. However, with Sinan's offer inevitably came a demand. Since claiming one of their more famous Christian scalps with the murder of Raymond II of Tripoli in 1152, the Assassins had been paying what amounted to protection money to the Templars at Tortosa, lest the Order press too enthusiastically into the mountains for retribution. As a token of reciprocal friendship it seems that Masyaf sought absolution from Jerusalem regarding this annual fee. We must assume that Amalric felt confident in acting unilaterally on this matter, as the envoy Abdallah departed with an answer in the affirmative for his master, a guide for the road and, crucially for the events which followed, a safe conduct granted and guaranteed by the king.

Whatever motivated the outrage which followed is still debated, but the facts of its execution are clear. On his journey back to Sinan at Masyaf, Abdallah was ambushed by a group of Templar knights, with one Walter du Mesnil meting out the fatal blow. Any hope of a Frankish-Assassin alliance died as surely as the emissary. William records the king as consumed by rage, apoplectic even. Elsewhere in his Historia we see Amalric as measured and considered, but his fury now is 'almost frenzied'. Knights of the royal household, no doubt made acutely aware of their lord's displeasure, rode to Sidon where Mesnil had taken shelter with his grandmaster, Odo de St. Amand. William's depiction of the guilty man is almost a diabolic caricature, right down to the missing eye. There at Sidon, declaring himself answerable to no earthly authority save that of the throne of St. Peter, and with the 'spirit of overweening arrogance', the Grand Master of the Templars refused to hand over a fugitive from the crown to the king's agents. Amalric, still seething in his 'violent anger', therefore set out himself to confront St. Amand. Finally, still at Sidon, the king seized Mesnil by force and cast him into the royal dungeons at Tyre. The crown and the Templars teetered on a precipice. Had Amalric lived longer, his desire to punish the Templars for this action against the Assassins is evident.

For a Templar-specific view on this see:

M. Barber, The New Knighthood: A History of the Order of the Temple (1994), pp.100-04.

For general overviews:

T. Asbridge, The Crusades: The War for the Holy Land (2012).

C. Tyerman, God's War: A New History of the Crusades (2006).

The Primary sourced used is:

William of Tyre, Historia rerum in partibus transmarinis gestarum.

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