Shaykhul Islaam Ahmad ibn Taymeeyah 728AH (May Allah have mercy on him) said,

وإذا كان في المسلمين ضعفوكان عدوهم مستظهرًا عليهم كان ذلك بسبب ذنوبهم وخطاياهم إما لتفريطهم في أداء الواجبات باطنًا وظاهرًا

“If the Muslims are weak and their enemies have gained an upper hand against them, then this is a result of their sins whether it is by their neglectfulness in establishing obligatory matters both inwardly (having the correct belief) and/or outwardly (like prayer, fasting, zakah, etc).”

Taken from: Majmu’ Fataawa 11/645