The Citadel. The Standard, #2

Great Britain is called upon by the American President to help neutralize an international terrorist threat. Escalating tensions within the Russian Federation lead experts to believe there is a conspiracy to destabilize its Western rivals: a mysterious arms smuggling gang is suspected of bringing weapons into the US for a series of terror attacks. Shanahan and Gawain are teamed with EUROPOL agent Lucretia Corcosa in an effort to infiltrate the ring, and assassinate its leaders.
They are drawn deep into the world of arms trading, Corsican mob and Russian espionage. Soon, the three have to confront the most dangerous enemy they have ever faced: The Citadel. The Citadel is a standalone novel, and can be enjoyed even if you haven't read other books in the series.