Complex sentence: Dmitriy, having now gained the full support of the Polish Commonwealth, formed a small army, comprised of approximately 3,500 soldiers from various private forces; with these men, he advanced on Russia in March 1605.
Split into simpler sentences: Dmitriy, having now gained the full support of the Polish Commonwealth, formed a small army of approximately 3,500 soldiers from various private forces. With these men, he advanced on Russia in March 1605.