ISIS and Syria

Open Email to House Speaker Boehner:

Dear Speaker Boehner,
According to the Washington Times, the House voted 273-156 Wednesday to back President Obama’s request for permission to train and money to arm the more reliable forces among Syrian rebels, giving the first congressional approval to the White House’s still-developing war plans.
What are we doing?
Syria has been engaged in a civil war for a couple of years; President Assad against the rebels. We don't like President Assad. He has been known to kill people who want to overthrow his government. He does not operate as a democracy. Does this give us some reason to overthrow him? Are we still in the process of nationbuilding?
Let's be more pragmatic.
Before the present Civil War, which was likely fostered by the US, Syria was a stable country for a great many years. We had no justification to upset it. It was as close to peace in the Middle East as one could hope to get. But, Pres. Assad now has two problems; he still has the rebels to contend with and now has ISIS, which is also his enemy.
It has been said that ISIS is a threat to the United States and must be defeated. Pres. Assad also wants to defeat ISIS. In other words, ISIS is a common enemy to both Assad and the US. Does it make sense to you that we should be supplying money and materials to Syrian rebels in order to destroy Assad's regime and leave us holding the bag of fighting ISIS in Syria alone?
Some may say that I don't understand the situation; it is more complex. I doubt that. But even so, if true, and I can't understand it, who can? The Middle East is a Mulligan stew. Should we be supplying money and materials to segments of combatants when we really don't understand the situation?
It seems simple to me. If ISIS is a threat to the United States and Assad wants to combat it, we should be helping him; not trying to throw him out of power.