Your Threat level with Russia should allow you to declare war on Russia. In the most recent expansion, that mutual Threat level with Russia would directly lower your Neutrality, which would then allow you to declare war on Hungary if Neutrality drops lower than your mutual Threat level with Hungary. In previous expansions, Neutrality was disconnected from Threat, so you used foreign spies to increase that nation's Threat level, and domestic spies to lower your own Neutrality. Now you can't lower your own Neutrality EXCEPT via Threat, or with the built-in "Prepare for War" exploit* in TFH. A lot of the manuals and documentation, and even some of the in-game tooltips, are obsolete.
Your options are: keep increasing Soviet Threat (via spies) until your Neutrality drops low enough to declare war on Hungary, use spies to increase Hungary's Threat directly, do both, or use the exploit.
* Exploit - Reduce your own effective IC below 30 (as in 38/29 - base IC / effective IC), and you should get a diplomatic Decision to "Prepare for War". This will gradually reduce your Neutrality over around 10 months by roughly 50 points. It may require a change of laws, different ministers, an increase in dissent, or all of the above to activate that reduction, which can then be changed back once the Decision is triggered.
