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Yesterday's research proved to them that Arcadia did not have what it takes to defend against the incoming threats.
Even with the best-case scenario of just half a dozen space cruisers coming to look for their lost space cruiser, it would still mean that they would need at least two dozen magus-level fighters. Not only that, but they would also need the means to defend against the opposing units' attacks and a way to shoot the ship down.
Of course, an all-out war would be incredibly risky for them. However, having two dozen magus-level fighters when the enemy units arrived would certainly put them in a better position. It might even cause the enemy to have second thoughts about attacking them directly.
If they could get lucky enough, they might even be able to make a deal with the opposing side to get some gains and avoid heavy losses.