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A precise measurement of the cross section of the process$e^+e^-\to\pi^+\pi^-(\gamma)$ from threshold to an energy of 3GeV is obtainedwith the initial-state radiation (ISR) method using 232fb$^{-1}$ of datacollected with the BaBar detector at $e^+e^-$ center-of-mass energies near10.6GeV. The ISR luminosity is determined from a study of the leptonic process$e^+e^-\to\mu^+\mu^-(\gamma)\gamma_{\rm ISR}$, which is found to agree with thenext-to-leading-order QED prediction to within 1.1%. The cross section for theprocess $e^+e^-\to\pi^+\pi^-(\gamma)$ is obtained with a systematic uncertaintyof 0.5% in the dominant $\rho$ resonance region. The leading-order hadroniccontribution to the muon magnetic anomaly calculated using the measured$\pi\pi$ cross section from threshold to 1.8GeV is $(514.1 \pm 2.2({\rm stat})\pm 3.1({\rm syst}))\times 10^{-10}$.

Precise Measurement of the e+ e- --> pi+ pi- (gamma) Cross Section with the Initial-State Radiation Method at BABAR