The barbaric terror attacks on the Belgian capital of Brussels highlight once again that the free world is fighting a global war against Islamist terrorism.

At least 34 people are dead in the wake of suicide attacks on the city’s airport and metro system. The Islamic State (ISIS) has claimed responsibility for the Brussels bombings, which follow the ISIS terror attacks on the heart of Paris last November that claimed 130 lives.

Troops have been deployed onto the streets of Brussels, and two Belgian nuclear power stations have been evacuated.

For several decades, Brussels has acted as a magnet for Islamist extremism, and ISIS and other Islamist groups have exploited the lack of border controls across Europe, as well as a striking absence of intelligence sharing throughout the European continent. Across Europe, governments are on high alert. In Britain, authorities have warned in recent days that a wave of ISIS terror attacks against London may be imminent.

Several thousand European-based Islamists have traveled to Iraq and Syria to fight with ISIS, and hundreds are believed to have returned to Europe, posing a major security risk. It is imperative that these foreign fighter pipelines are closed down and that more stringent border security and immigration policies are put in place throughout Europe.

Until European countries have full control of their own borders, however—impossible under the current Schengen Agreement—it will be difficult for European governments to stem the flow of Islamist terrorists across the continent. Sovereignty matters, and the European project and the centralization of political power within the European Union have made it far harder to fight and defeat Islamist terrorism.

This is not a time for leading from behind from the White House.

This is also not a time for leading from behind from the White House. This is a moment for robust American leadership on the world stage and a reaffirmation of the transatlantic alliance. The United States and its allies must work together to combat the vast network of Islamist terror cells across Europe, and be prepared to emphatically defeat ISIS in the Middle East and North Africa.

It is imperative as well that Barack Obama recognize the ideological nature of the enemy we face. The free world is engaged in a global fight against Islamist terrorism, not simply “violent extremism,” as Obama likes to call it.