I believe you're misreading Iran's intentions.

It is certainly the case that Iran has been a considerable supporter of Hezbollah in Lebanon and of the Bashar Al-Assad regime in Syria. But in each of those cases, Iran’s support is more an expression of its myriad security dilemmas in the region than its desire to become a hegemonic power.

The rationale behind the Iran-Syria relationship straightforward: to avoid domination by other states in the region. In the case of the Assad regime, this has meant ensuring its survival against other Arab regimes with rival ideologies (such as Saddam Hussein’s Ba’ath Party) and securing its strategic interests in Lebanon and vis-a-vis Israel (especially regaining the Golan Heights). For Iran, Syria was an especially important Arab ally during its war against Iraq, providing it with crucial diplomatic and military support at a time when nearly all Arab states backed Saddam. Syria has also been an important facilitator of Iran’s support for Hezbollah and Hamas. The reason why Iran is weary of the idea of letting the Syrian government collapse (but is open to letting Assad go) is because Iran believes a new government would be decisively anti-Iran, which it undoubtedly would be.

There is a plausible question about Iran’s increasingly aggressive behaviour in all these territories since at least 2005, not to mention the expansion of its nuclear program over the past decade. But this behaviour would only be worrisome if we ignore the impact of the Afghanistan and Iraq wars on Iran’s national security calculus. Encircled by a ring of instability and several regional powers and one superpower that repeatedly call(ed) for Iran's destruction, and no doubt bewildered and emboldened by the sheer incompetence of American post-war planning, in equal measure, Iran set about fortifying its interests and positioning itself as a stakeholder in the future of the region.

However menacing these actions may in retrospect appear, they are not by any means indicative of a determination to reconstruct the Persian empire. As even a most perfunctory accounting of the Islamic Republic’s regional behaviour would show, it is the wish to mitigate a precarious security environment, and not naked imperial ambition, that drives Iran’s strategic thinking in the wider Middle East.

For more than three decades, Iran's proxy partnerships have helped it push back against hostile initiatives - e.g., US military intervention in Lebanon, Israel’s occupation of southern Lebanon, Saudi-backed expansion of Taliban control in Afghanistan, Saddam’s antagonism toward the Islamic Republic, US occupation of Iraq - that threatened Iran’s strategic position. They have also enabled Tehran to reduce the chances that nearby states - Lebanon, Afghanistan, post-Saddam Iraq, Bahrain (where America’s 5th Fleet is based) - will again be used as platforms to attack the Iran or otherwise undermine its security and independence.

We see that this strategy, however painful, has in the long-term been successful. The threat of an attack on Iran is nil, while Iran's strategic depth is as great as ever in its modern history. Western powers have accepted Iran's nuclear program and, more importantly, its standing in the region with a comprehensive nuclear deal to be signed in a week or two.