The Houthis have stepped up attacks on Saudi Arabia’s airports, and Thursday’s deadly strike on Riyadh is the worst in years. Here is what is driving the escalation.

The Houthis are an Iran-aligned group that controls much of Yemen. They are fighting a Saudi-led coalition that backs Yemen’s government. The latest strikes come as the US-Iran war grinds on, with Tehran’s allies pressing Saudi Arabia.

Airports are strategic targets. Hitting Riyadh airport disrupts air travel and sends a message days before the Saudi capital hosts a major meeting of global oil chiefs. The Houthis have also warned airlines to avoid Saudi airspace.

That warning is being taken seriously. Europe’s aviation safety agency has widened its advisory, saying the Houthis have shown they can strike deep inside Saudi territory. An aviation risk group, OPSGROUP, told members to avoid Saudi airspace entirely.

Saudi Arabia has responded with airstrikes on Sanaa. Its coalition destroyed three Houthi missile launch platforms. The risk now is a cycle of strikes that further threatens flights, shipping and oil markets.