It's a price tag that has obsessed US President Donald Trump since his first term: the cost of the military shield the US provides to its allies. Particularly in Trump's sights are South Korea and Japan. Both countries pay around €1 billion per year for American protection on their own soil, a sum deemed insufficient by the US president. Last October, Trump even claimed that South Korea should pay 10 times more. Is the US president serious, or is this another high-stakes bluff? FRANCE 24's Adam Hancock, Alexis Bregere, Mélodie Sforza and Chloé Borgnon report.