Napoleon would end up leaving Iberia in order to deal with the Austrians in Central Europe, but the Peninsular War continued on long after his absence. He never returned to Spain after the 1808 campaign. Several months after Corunna, the British sent another army to the peninsula under the future Duke of Wellington. The war then settled into a complex and asymmetric strategic deadlock where all sides struggled to gain the upper hand. The highlight of the conflict became the brutal guerrilla warfare that engulfed much of the Spanish countryside. Both sides committed the worst atrocities of the Napoleonic Wars during this phase of the conflict. The vicious guerrilla fighting in Spain, largely absent from the French campaigns in Central Europe, severely disrupted the French lines of supply and communication. Although France maintained roughly 300,000 troops in Iberia during the Peninsular War, the vast majority were tied down to garrison duty and to intelligence operations. The French were never able to concentrate all of their forces effectively, prolonging the war until events elsewhere in Europe finally turned the tide in favor of the Allies. After the invasion of Russia in 1812, the number of French troops in Spain vastly declined as Napoleon needed reinforcements to conserve his strategic position in Europe. By 1814, after scores of battles and sieges throughout Iberia, the Allies had managed to push the French out of the peninsula.