Who actually does: the media, weapons companies, the permanent government, presidents (including simply by decreeing a "war on terror", through misspending, lying, simply acting, signing treaties), political parties, culture (the one Biden lives in, in which Israel's sovereign right to attack Iran is uncontroversial), soldiers who obey illegal orders and the culture that leads kids to that place.
Who should decide: we the people of the world, through democratically created and enforced international and national and state laws.
This is complicated by another question: what does the law say? But we may give too much importance to that. Laws violated for decades can be as hard to enforce as laws not written yet. But getting old laws enforced and new laws created has to be part of our strategy.
With a better communications system and election finance system and party system and voting system and congressional checks on presidential power -- in other words with everything we'd need in order to control Congress and create public referenda on wars or anything else -- we might see wars blocked by referenda. But if we're making exceptions for defensive (legal) wars, then why vote the illegal ones up or down, why not (given our hypothetical ability to control Congress) deter war criminals through impeachments, including the impeachment of attorneys general who refuse to prosecute them?
And why weaken the already unconstitutionally weak War Powers Act in the manner proposed this morning by Morton Halperin and taken from "Deciding to Use Force Abroad" by the Constitution Project, which effectively would limit Congress to deciding on whether or not to fight each aggressive war? Why not ban all aggressive wars by enforcing the laws that already do that? Or why not leave Congress the undiluted power to decide on all wars, as spelled out in the Constitution? I'm just asking. If you have to have a war powers act, why not at least limit the exceptions to congressional approval to cases of immediate defense of the United States? And then, what other wars should there be?
Through counter-recruitment including countering mercenary recruitment.
Through systemic reforms (including restoring all but the 3rd amendment -- the one still left standing; and think about why that is).
By forcing our representatives in the House to reclaim and accept power on our behalf that they do not want. By forcing them to use the power of the purse. By forcing them to use the powers of impeachment and inherent contempt. (Barbara Lee was right on Thursday to introduce a bill to block funding an escalation, but so was John McCain the same day to advocate Congress bringing a general in for questioning. We need testimony from current war makers and from recent criminals.)
By holding war criminals accountable through national, state, local, civil, foreign, or international suits.
By exposing the lies that Congress debated 7 years ago this week.
By restoring to Congress the power to make laws, wars, treaties, appointments, and spend all public money.