In an op-ed published by the LA Times, Ya'alon challenges the common argument by US political leaders that destroying Islamic State is America's top priority in the Middle East.
Instead, he writes, that's not nearly as important as confronting the challenge posed by Iran. This is a summary of his arguments.
The nuclear deal that went into effect a year ago may have postponed the danger of an Iranian nuclear bomb, but the multifaceted threat of a militaristic, messianic Iran - 80-million strong - is much more menacing to Western interests than the Sunni thugs and murderers of Raqqa and Mosul.
From Tehran's perspective, it gained much more than it gave up in the deal. In exchange for postponing its military nuclear project, it achieved the lifting of many economic sanctions, an end to its political isolation, and the loosening of restrictions on its ballistic missile program.
The deal was negotiated by an UN-designated team of leaders from six global powers, the so-called P5+1.
Ya'alon says the P5+1 held an exaggerated fear of taking any steps that might give the Iranians an excuse to scuttle the deal. As a result, Tehran won wide latitude to advance its influence throughout the region, as it no longer fears a US-led "military option."
What should be done now?
Concerned nations need to work together to prevent Iran from exploiting the nuclear deal in order to redraw the political map of the Middle East in its favor.
This means taking steps to ensure strict inspection of Iran's nuclear facilities - and not just by the International Atomic Energy Agency. After all, the vast majority of Iran's nuclear violations were exposed by Western intelligence agencies, not the IAEA.
It is not too late to repair the impression that the West views Iran as part of the solution to the problems of the Middle East, rather than the chief source of the region's instability and radicalism. Those who believed that the nuclear agreement would lead to a more moderate, open, reformist Iran, at home and abroad, regrettably suffer from wishful thinking.