Saudi Arabia has financed a large purchase of infantry weapons from Croatia and quietly funneled them to antigovernment fighters in Syria in a drive to break the bloody stalemate that has allowed President Bashar al-Assad to cling to power, according to American and Western officials familiar with the purchases. The weapons began reaching rebels in December via shipments shuttled through Jordan, officials said, and have been a factor in the rebels’ small tactical gains this winter against the army and militias loyal to Mr. Assad. Washington’s role in the shipments, if any, is not clear. Officials in Europe and the...