Complex sentence: The song, the band's first single in the US, started off modestly in Europe, but upon release in the US, steadily grew until, after 32 weeks on the chart, topped the US "Billboard" Active Rock chart.
Split into simpler sentences: The song, the band's first in the US, started off modestly, but upon release in the US, steadily grew over time. After 32 weeks on the chart, the song topped the US "Billboard" Active Rock chart.