Breaker Immediately Trips, Need Help Troubleshooting Why
Shared Circuit
Assuming this isn't just some crazy coincidence, I'd bet on:

The alarm and the lights are on the same breaker
When you left the breaker off for an extended period of time, the battery on the alarm started to get low, which caused the chirping as a (correct) warning
Red and black on the alarm are battery connections and not incoming AC power. A typical alarm system has a converter (either a wall wart or built in to the main alarm panel) to convert from 120V AC to 6V or 12V DC. That AC wiring will, in the US, almost always be black &amp; white (hot &amp; neutral). Black &amp; red is, however, typical for battery power. A battery, even when relatively low, can make quite a spark when shorted.
Sensors, bells, etc. connected to the alarm have nothing to do with the current (pun intended) problem.

So what is going on? Anything is possible. But since the breaker is tripping immediately, the likely scenarios are:

Something came loose &amp; shorted when you put the new wires, dimmer, etc. back in the box. To test this, open up the box and disconnect the hot &amp; neutral from the dimmer and see if the breaker can be reset. If so, you know the problem is somewhere in: dimmer, lights, connections between dimmer &amp; lights.
Something came loose &amp; shorted when you messed with the alarm. Not on the low voltage side. You need to find the incoming power - either to a wall wart or into the main alarm panel - and disconnect it and then try resetting the breaker.

If you can't find the problem in one of those two places then you have to disconnect each light or receptacle on the circuit until you find the problem. There is a small but non-zero possibility that somewhere in the circuit something else (e.g., a bad backstab connection) went bad coincidentally with the new work.