Saudi Arabia has carried out more than 50 airstrikes against Houthi positions in Yemen. The targets were strategic sites that the Iran-backed rebels hold in the capital, Sanaa. The bombardment followed a decision by the Houthi movement to completely close a strait regarded as the world's second most important oil export route. Blocking that passage cuts off a corridor used for a large share of global oil and fuel shipments. For Saudi Arabia, the strikes amount to a direct military answer to the closure. The scale of the response suggests Riyadh is treating the move as far more than a temporary provocation.