The arrest of Ben-ami Kadish on charges of divulging nuclear and other military secrets to Israel during roughly the same period when Jonathan Pollard was doing the same will, no doubt, lead to a revival of the claim that spying for an ally is not a serious offense. Here I'll simply say for the record that this is a wildly implausible notion.
We can grant that spying for an ally is not as bad as spying for an avowed enemy. Unlike the enemy, the ally can generally be expected not to use the stolen information to the direct detriment of the victim country, because the interests of the victim country and the ally generally overlap. But the key word here is "generally." The interests of the United States and Israel (or any ally) are not identical, which is why, even though we share some intelligence with Israel and other allies, we don't share all of our intelligence, which in turn is why Israel needed Pollard and (apparently) Kadish to get these particular secrets.
Let's be concrete. Israel in the 1980s faced, and continues to this day to face, existential threats from some of its neighbors. Under these circumstances, it may well be a sensible policy to obtain as much weaponry as possible, both for actual use and for deterrence. The cost-benefit calculus for Israel in deterring and neutralizing attacks may justify taking the risk of an escalating Middle Eastern arms race and the risk that Israel's enemies (and their supporters, which during the relevant period included the Soviet Union) will learn about the vulnerabilities of U.S. weaponry. (U.S. military brass opposed to Pollard's release also point to the fact that secrets he stole made their way to China.) For the U.S., even if we consider the survival of Israel an extremely important foreign policy priority, the risk analysis can come out differently. To repeat the obvious, that's why the U.S. doesn't simply share all of its weapons secrets with Israel and other allies.
There is also, of course, the possibility that today's ally will be tomorrow's enemy. Think of the Soviet Union during WWII. To be sure, this example is not an exact analogy. The WWII alliance with the Soviet Union was never a close working relationship, given that it was born of a lesser-of-two-evils calculation, while the U.S. and Israel have enjoyed friendly relations (if not always identical policy objectives) since the latter's establishment 60 years ago. Nonetheless, if we allow a defense of "I was spying for an ally," we can well imagine that all spies will make their own rationalizations about how trustworthy particular allies are.
Posted by Mike Dorf