IfW analysis: Replacing US aid to UA would cost Europe only around 0.12% of GDP. Replacing the US contribution to conventional NATO deterrence, on the other hand, would require around 300k additional troops. Likely requires 3.5% GDP defence spending, rather than 2.

https://www.ifw-kiel.de/fileadmin/Dateiverwaltung/IfW-Publications/fis-import/4f1009ce-a53d-4043-bda1-92f60a71f389-KPB_183_en.pdf