Or are they usually are? Take for example between US and its allies. In modern days, are there any safe guards for them to be kept classified. Or safeguards to be kept transparent?
Take for example: US selling bunker buster bombs to Israel. We only know about the deal after it has concluded.
Lets look at another example: Turkey's acquisition of the Russian S-400 system. As Turkey is also a potential customer of US arms, what would happen if they kept their Russian weapon acquisitions secret? Do they have the option too? Or could they break the agreement after they have received their F-35 (fighter jets) and had them transferred out to Turkey soil, for example?