To me, a question like "Will anything attack our kingdom this week?" would be sufficient to yield an answer, but to encourage the players to be more specific, I'd do something like the following:
Players: "Will anything attack our kingdom this week?"
Ok, so now they know that something will attack their kingdom, but little do they know, it's just some birds not native to the region, on their normal migratory cycle, flying through the edge of the kingdom's territory, and they happened to land a few droppings on some unlucky merchants.
So specific-enough questions like that may yield an answer, but likely one that's not helpful. They may know something will attack their kingdom, but as for what, and in what way... that's not what they asked for, so why should I tell them? If you do it this way, the limitation you've imposed would also protect you from having to answer a follow up cast of Divination, asking "What will attack our kingdom this week?" or "When will they invade?". Much in the same way you won't tell them where the Eye of Ragnarok is hidden, you wouldn't tell them who the invaders are, or what their plans are either.
On the other hand, if the players ask something like "Will the kingdom of Alderkar attack Sandport this week?" That's MUCH more specific, and would therefore yield a much more useful answer.
If you wanted to, however, you could also use a vague question like that to give the players a hint at an upcoming story arc, via the "cryptic rhyme or omen" part of the spell description, allowing you to send your players on a new adventure filled week of investigation, plots, corruption, scandals, and combat, trying to figure out the details such as:
- Who's attacking?
- When will they invade?
- What will they invade with?
- How much devastation will there be?
- Who's involved?
- What can we do about it?
Yet another option would be that you can turn a vague Divination into a self-fulfilling prophecy, that's ultimately harmless to show your players that they can't keep doing this.
To answer the question itself: Yes, that particular question is "specific enough", though the answer you get would be all but useless. And I could see how the goal of the spell would be to protect their citizens, but no good ruler would rely on one source, and one source only to give them all the answers. A good ruler would bolster their kingdom's defenses anyway, making it stronger against whatever may come their way, and to form alliances with neighboring kingdoms, and to make his people happy to help reduce potential threats.
For future reference on the Divination spell: If it were me, I'd say, the more specific the question, the more helpful the answer, with the reverse being true as well, unless you needed a way to send your players on a new quest, in which case, you can give them a cryptic prophecy, Percy Jackson style, to send them in the right direction.