I dont see how that would antagonize allied kingdoms when all Haruto did was just defending his own border. There are lies you can tell to justify your own king being captured at another country's borders, but even then those are hard to sell.
If king is valuable enough, you get to cripple their military for a few years. If he isnt, you get to keep him prisoner and buy a few years politically as the replacement/regent needs time to take over the mantle.
Depending on how you spin it, it is quite easy to twist into a justification for war as the king didn't declare war. He ust went to the border with his army. This can be argued as a defensive measure as it could be seen as him making sure that an unstable nation (reminder that he JUST got news that the monarchy had collapsed) doesn't act out in aggression. So him being held at ransom could be seen as confirmation that the nation Haruto is a part of is unstable and dangerous.
The assumption that the kingdom wouldn't immediately attack is predicated on them not having back ups in place and or them not being reckless. Considering that they tried to attack as soon as they saw a weakness shows that they are highly aggressive which suggests some recklessness.
Even if they don't attack directly, there is prevent them from trying to use agents to cripple the queen. (Sure they would be able to undermine the country anyway but capturing the king gives them even more motive to act).