What can they realistically do? Send German troops there? It won't happen ever, we both know it. Shutdown Nordstream 2 earlier? It's unlikely to have any effect. Direct supply of weapons into Ukraine? Earlier and preemptive sanctions? None of that will deter Putin, in fact it will probably just encourage him to act faster.

You also have to take into consideration the last 2 times Germany entered a conflict it didn't work out too well for them, so they have all the more reasons to be reserved.