xNation cannot rally it's national morale is too low

[aprezto]

I am quoting this note you get in the event log from memory, so it is not verbose, but hopefully you know the message I mean.

What does it mean?

[Russian Guard]

It means that the Nation sending the message has very low NML, so if you are allied to them - and have placed a "Rally" location for them militarily - they will not comply. They are very likely holed up in their Capital, or trying to retake the Capital. But in any event, Nations with low NML usually ignore Rally requests.

[Dab]

To elaborate:

Go to your military screen and any nations you are allied with can be selected in the bottom left corner - this also shows you a rating of how strong the alliance is.

You can set a rally location for your ally and they will try to move their armies to the province you select if their national morale is high enough and their attitude is good towards you. One kind of 'cheating' strategy is to send an allies armies across the map; say from Spain to Moscow; the Spanish armies will be decimated through attrition and battle, then you can break the alliance and attack them...

Also, your ally won't start to move until the second turn after you set their rally location and setting a rally location uses 1 diplomatic action for the turn.

As far as the mesage goes, I think it appears in the events report even if the nation reporting doesn't have any allies. An opportune time to attack a nation is when they are reporting low national morale.