A Project Manager’s job is to get their project done. On time and within budget. Period.
This is a trend I’ve seen in many places. The project manager fights for their project to a fault. They get the money and resources they need. They win the battle of conflicting priorities for shared resources. They take no prisoners and get things done. While in the process of doing it, though, sometimes they rob other projects and processes of resources, and that ultimately hurts the company significantly.
As a project manager, you can never lose sight of the fact that you work for the company, not the project. When there is a scheduling conflict, you have to ask questions and find out what the conflict is and what the importance of the other item is to the company. There are ultimately times when you should stand down and let the other people through.
Your stakeholders can sometimes make this a delicate balancing act. They will not always agree that the other project should be let through. Not everyone thinks in terms of the Company as a whole. You should take the time to explain yourself, though, and if you find yourself and what you believe is the correct thing to do in conflict with your stakeholders or, worse, project sponsor, take the time to escalate. Don’t do it in a contrary manner; simply consult the right authority in the company and ask which choice takes priority. Once you do that, report your findings and acquiesce to it. Refer to the higher decision and seek help from leadership in explaining things if necessary.
Doing the right thing for the company is not always good for your project, but ultimately it is the right thing to do for your career. Demonstrating to leadership that you can think globally about the company will ultimately help your career.