US combat forces constitute approximately 5% of European combat power.

Omissions

• The claim relies on the undefined concept of 'combat power,' which is a qualitative military assessment encompassing technology, training, readiness, logistics, C4ISR, and other enablers — not a standard statistical indicator.
• The Bruegel report indicates that US forces have substantially higher combat power per soldier than European forces, which would mean the US share of combat power is likely higher, not lower, than its share of personnel.
• No primary or secondary source was found that explicitly states US forces represent 'around 5% of European combat power.'
• Two independent sources or one primary source confirming the claim could not be found.

Sources

• SecondaryBruegelThe combat power of 300,000 US troops is substantially greater than the equivalent number of European troops distributed over 29 national armies. A realistic estimate may therefore be that an increase in European capacities equivalent to the fighting capacity of 300,000 US troops is needed.
• SecondaryWTTW News (PBS Chicago)There are usually 80,000 to 100,000 troops on the continent, with more than 36,000 in Germany.
• SecondaryFortuneAmerican troops pack more punch than European ones, and fighting Russia without the U.S. would require adding more than 300,000, think tank says.