Without a credible defence
French and UK nukes, maybe EU nukes. Focus on strong Anti-Air, Anti-Missile and Drone capacities and Missiles/long range Artillery for instance.
and a strong economy
Which needs good infrastructure investments, good education investments and good social security to protect local consumption. All these points will be jeopardized by spending on military instead.
If the war in Ukraine wasn’t enough to convince you
The war in Ukraine shows that a land invasion in the 21. Century does not go well without massive troops and massive losses. Meanwhile EU countries paid more money for Russian Oil and Gas since the full scale invasion than Ukraine received in aid. If the EU had invested more in renewable energies that would have helped Ukraine by never filling the Russian war chest in the first place.
It’s also not wasted money if you invest in the domestic defense industry
If the products are needed and the spending is efficient. Something that at least for Germany has often not been the case and very little showed for absurd amount of spending.
And we’ve spent at this level before without invading people.
Which countries specifically at which times? UK and France have been invading and occupying well into the 60s and projecting military power in "friendly occupations" to steal resources until today, e.g. see West Africa.
For Germany i found the highest spending to GDP ratio at almost 5% in the 50s and 60s in western Germany. At that time western Germany was bankrolled by the US to ensure a "bullwark" against Communism. This would not have been possible standing on its own feet.
But if you think it always leads to invasions, you should be even more worried about increased military spending and military industrial capacity in Russia
Russia is already invading. That is why they spending is so high. They do hope to finance it later by taking Ukrainian resources. If you think we should mimic Russia, whose resources should pay for our armies? Ukraine? Or should we invade Russia to secure their resources?
EDIT: Also we must not forget that the EU has almost triple the population of Russia. To defend against a possible Russian threat requires more EU unity and integration, rather than more individual military spending. But for such an integration you need to invest politically, economically and socially.