Final answer:
Malcolm's statement means that those who serve with him are not fully committed to the cause and their hearts are absent.
Step-by-step explanation:
The line you mentioned is from a poem by Stephen Crane called 'War is if Inasmuch as' where Malcolm is referring to the people who serve with him but are not fully dedicated to the cause. He is saying that those who serve with him are compelled to do so and their hearts are not fully in it. They are like 'constrained things whose hearts are absent'. This means that they are not truly committed to the cause and are only serving out of obligation or external pressure.